Type about:config into your address bar and you will be brought to the about:config settings. This is a powerful way to tweak your settings in ways that are not normally accessible through the Options menu. By simply double clicking one of the available options (after copying and pasting them into the filter bar to find them easily), changes can be made, including the following:
- browser.block.target_new_window — if set to true, links that normally force a new window to open will open in the current window instead.
- browser.xul.error_pages.enabled — if set to true, Firefox displays an error page similar to IE instead of a message box if loading a page fails.
- layout.frames.force_resizability — if set to true, allows the user to resize frames on any web site that uses them.
The following changes can be made to speed up browsing. Normally the browser will make one request to a web page at a time. When you enable pipelining it will make several at once, which usually speeds up page loading. Make these changes to enable pipelining:
Set "network.http.pipelining" to "true"
Set "network.http.proxy.pipelining" to "true"
[Tip: If have your browser set to connect to an optional http proxy, you can leave "network.http.pipelining" setp to "false", then add the domains of websites that don't work with pipelining (like images.google.com) to the proxy exclude list.]
Set "network.http.pipelining.maxrequests" to 8. This means it will make up to 8 requests at once rather than the default of 4. This is only an advantage if you have a reliable internet connection that isn't particularly slow.
Finally, right-click anywhere and select New-> Integer. Name it "nglayout.initialpaint.delay" and set its value to "0". This value is the amount of time the browser waits before it acts on information it receives, but it will increase the total time taken to render the page. This option is more suitable for a faster computer ("250" is the default). Try a value of "100" if "0" causes problems.
For broadband users:
Set "network.http.max-connections-per-server" to 14. Many guides recommend setting this figure to 100, but this can have undesirable effects upon webservers.
Set "network.http.max-connections" to 48
A much more complete list can be found here, with descriptions and which values the preference will take (where applicable)
Editing the userChrome.css File 
userChrome.css is a file that allows you to change the appearance of Firefox with CSS rules. The actual browser window (i.e., not the webpage, but everything else) is called the "chrome". The file userChrome.css overrides default settings to allow for more customization.
userChrome.css is not created by default. It should be created in your profile folder, which can be found in the following places:
Windows: %appdata%\Roaming\Mozilla\Firefox\Profiles\<Profile name>\ Linux: ~/.mozilla/firefox/<Profile name>/ Mac OS X: ~/Library/Mozilla/Firefox/Profiles/<Profile name>/ or ~/Library/Application Support/Firefox/Profiles/<Profile name>/
In all cases, the profile is randomly named, with 8 characters followed by .default. Inside that folder is another named "chrome". Inside the chrome folder is where userChrome.css needs to be created.
Editing the userContent.css File 
- Allowing extension downloads from mozilla.org instead of www.mozilla.org will match all subdomains of mozilla.org, just as blocking cookies from doubleclick.net instead of www.doubleclick.net will allow blocking of all doubleclick.net subdomains. Allowing "www.mozilla.org" would not allow "addons.mozilla.org". However, as a security measure, when whitelisting sites for extensions, the most specific domain available should be used (i.e., addons.mozilla.org instead of mozilla.org) to prevent potentially malicious installs from other subdomains on a site.
- To get Firefox on a computer without an internet connection or where internet downloads are blocked you can download the file as normal form another computer (usually you can do this at a public library) then put the *.exe file on some removable media and transfer it to the first computer.