To control the built-in HTML5 player:

• In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.
• In the search box above the list, type or paste media*play and pause while the list is filtered
• To delay media from auto-starting in background tabs, I suggest: double-click the media.block-play-until-visible preference to switch the value from false to true
• To prevent media from auto-starting in any tabs: double-click the media.autoplay.enabled preference to switch the value from true to false

• For HTML5 media you can try to set the media.autoplay.enabled pref to false on the about:config page.
• Try to set the Boolean pref media.windows-media-foundation.enabled to false on the about:config page.
• Type in the address bar: about:config Then search for: plugins.click_to_play Double click and it will be set to true) - doesn't stop videos from playing either. Sent feedback to FireFox about the issue.

• Find layout.frame_rate.precise set it to enable. It increases animation speed. On my system it elliminated choppy scrolling almost completely.
• Find privacy.trackingprotection.enabled and set to true. It's a built in Firefox tracking protection. It also speeds up page loading by 20-40%. This option replaces Disonnect extension and it may be removed after enabling this fix.
• To do so, search for image.mem.max_decoded_image_kb. By default, this is set to a very high value – 256000 or ~256 MB. Reduce this cache to something smaller, such as 51200 (50 MB).
• To reduce memory consumption by Javascript, locate the parameter javascript.options.mem.max and change its value from -1 (automatic) to 51200 (50 MB).
• Also find javascript.options.mem.high_water_mark, and set it to 30 from the default 128. This parameter tells the garbage collector to start running when javascript is using 30 MB of memory. Garbage collection releases memory back to the system.
• Alternatively, you can reduce the number of pages Firefox keeps in cache. To do this, open about:config and set the desired value in browser.sessionhistory.max_entries. A value of 10 should do.

After some digging, I found out that this behavior is controlled by a parameter that you can access through typing “about:config” in the address bar. This parameter is called browser.sessionstore.interval.

It is set to 15 seconds by default. In my case, I reset it to a more sane (at least for me) 30 minutes. Since then, I’m only seeing about 2GB written to disk when my workstation is left idle, which still feels like a lot but is 5 times less than before.

https://groups.google.com/forum/#!topic/alt.comp.freeware/Mv3g_MSQyq8
Since Firefox 3.5 this preference is superseded with setting:

browser.sessionstore.max_tabs_undo and
browser.sessionstore.max_windows_undo to 0.

• uBlock Origin (replace AdBlock Plus)
• AdMuncher
  
• Better URL Bar
• Flash Control
• Mouseless Browsing
• Dark Background and Light Text
  
• Night Mode Pro
• Owl
  
• Theme Font & Size Changer
• HTML Content Blocker
  

• CompactHeader
• ConfirmFolderMove
• Header Tools Lite
• Join-NG
• Manually Sort Folders
• Quick Folder Move
• Right Encoding

• ... For example, if your system's DPI is 144, then the value to add is 144/96 = 1.5. Changing layout.css.devPixelsPerPx (via about:config) from -1.0 (default, step of 0.05) to 1.5 makes web page contents use a DPI of 144, which looks much better.

• network.http.version = 1.1
• network.http.pipelining = true
• network.http.proxy.pipelining = true
• network.http.pipelining.ssl = true
• network.http.pipelining.aggressive = true
• network.http.pipelining.maxrequests = 8
• You can go about:support to verify settings.

• You can set the pdfjs.disabled pref to true on the about:config page to disable the build-in PDF viewer and use the Adobe Reader instead.
• ...if you want to switch back to the plugin you've used before to view pdfs just go to firefox options > applications & define the default action for the portable document format (pdf).

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. Right-click somewhere in the list and select "New > String"
4. For the name of the preference type browser.cache.disk.parent_directory
5. For its value type the path to where you want to store the cache
6. Next locate the preference browser.cache.disk.enable, it must be set to true, if it is not, double-click on it to change its value

There is a known issue with the Mozilla Firefox web browser that prevents the ReadCube HTML5 Web Reader from loading on some computers. In particular, Windows and Linux systems are known to be affected. If Firefox is not able to correctly recognize your graphics card for enabling hardware acceleration, a feature called DirectWrite is disabled.

1. In the address bar, type about:config and press Enter
2. Search for the preference name: gfx.font_rendering.directwrite.enabled and set the value to true
3. Search for preference name: gfx.direct2d.disabled and set the value to true
4. Restart Firefox
5. These two settings will disable anti-aliasing for downloadable fonts. So you might want to set your own replacement fonts.

Source: http://support.readcube.com/knowledgebase/articles/402128-firefox-issue-poor-font-rendering-in-html5-reader

Note: Enabling DirectWrite will cause the built-in PDF reader of Firefox to show blank pages.

Related bugs:
• Bugzilla Bug 1125461
• Bugzilla Bug 923612

• browser.cache.disk.enable = false
• browser.cache.memory.enable = true
• browser.cache.memory.capacity = -1
• Go about:cache to verify settings.

You can add the following two string keys via about:config instead of editing prefs.js directly.

# change location of bookmark file:
user_pref("browser.bookmarks.file", "d:\\manwai\\bookmarks.html");

# For Firefox 3.0 and after...
user_pref("browser.bookmarks.autoExportHTML", True);

/* Attachment pane at bottom of message window */
#attachmentView {
  -moz-appearance: none !important;
  height: 40px !important;
  overflow: auto !important; }

/* hide go button */
#go-button {
  display: none !important; }

#mail-menubar > menu {
  font-size: 12pt !important
}
#compose-menubar > menu {
  font-size: 12pt !important
}
statusbarpanel {
  font-size: 12pt !important
}
menupopup > * {
  font-size: 12pt !important
}
#urlbar {
   font-family: monospace !important;
   font-size: 13pt !important;
}
#throbber-box {
  display: none !important;
}
#folderTree {
  font-size:   12pt !important;
}
#threadTree {
  font-size:   12pt   !important;
}

treechildren:-moz-tree-cell-text(read) {
  font-size: 12pt !important;
}

treechildren:-moz-tree-cell-text(unread) {
  font-size: 12pt !important;
}

#msgHeaderView {
    max-height: 15em !important;
    overflow: auto !important; }

#header-box {
    font-size: 12pt !important;
}
#subject-box {
    font-size: 12pt !important;
}

.moz-text-flowed, .moz-text-plain, .moz-text-html {
  font-size: 12pt !important;
}

body {
	font-size: 12pt !important;
}

/* Disable horizontal bars beside quoted texts */
blockquote[type=cite] {
	padding: 0 !important;
	border: 0 !important;
}

user_pref("mail.server.server1.max_cached_connections", 5); 

# show user agent:

user_pref("mailnews.headers.showUserAgent", true);

# use default system color:

user_pref("browser.display.use_system_colors",truye);

#http://www.hmetzger.de/net6e.html#7
#
# use traditional display of quotes
user_pref("mail.quoted_graphical", false);
user_pref("mail.quoteasblock", false);
user_pref("mailnews.display.disable_format_flowed_support", true);

# pretend to be a netscape 4
user_pref("general.useragent.override","Mozilla/4.0 (compatible; MSIE 6.0; Windows 98)")

# stop format=flowed when reading messsages
user_pref("mailnews.display.disable_format_flowed_support", true);

# stop format=flowed when sending messsages
user_pref("mailnews.send_plaintext_flowed", false);

# change the attribution line
user_pref("mailnews.reply_header_type", 2);
user_pref("mailnews.reply_header_authorwrote", "%s spoke thusly");
user_pref("mailnews.reply_header_separator", " ");
user_pref("mailnews.reply_header_colon", "
");

# duping all startup options and values
netscp6.exe -help > c:\ temp.txt

# wrap long line
user_pref("view_source.wrap_long_lines", true);

# minimum size of fonts
user_pref("font.minimum-size.x-western", 13);

#disable download plugin prompt
user_pref("plugin.display_plugin_downloader_dialog", false);

This tip requires (basic) knowledge of CSS. All settings are done by
manipulating userContent.css. The main document describing the commands
can be found here. I will only give some examples now, to show the power
of Mozilla... :-)

# These makes the font in the mail/news window bigger (usercontent.css):
in this example 12 points.

.moz-text-plain pre {
font-size: 12pt;
}

# Use another font for mail/news display:
.moz-text-plain pre {
font-family: Arial ! important;
}

user_pref("mailnews.start_page.enabled", false);
user_pref("browser.wfe.ignore_def_check", true);
user_pref("mailnews.start_page.url", "about:blank");

http://gentoo-wiki.com/TIP_Integrate_Thunderbird_and_Firefox
user_pref("network.protocol-handler.app.ftp","/usr/bin/firefox");
user_pref("network.protocol-handler.app.http","/usr/bin/firefox");
user_pref("network.protocol-handler.app.https","/usr/bin/firefox");