Mozilla Firefox is a slow and sluggish web browser from the Mozilla Corporation which is getting worse, less responsive and more useless with each new release.
 Release history
One can, as a general rule, say that Firefox versions after version 10 are useless trash which should be avoided. Use Firefox 10 or below or use another browser.
Firefox lets you make profiles. These are different browser configurations stored in separate directories under ~/.mozilla/firefox.
Profiles are very useful. You can, for example, have one profile for normal browsing, one for banking, one for web development and one for web site testing. Addons added to one profile does not show up in another, and the different profiles can have different themes (black for one thing, white for another, and so on).
 Useful extentions
Extentions is part of what makes Firefox an alright browser to have installed. Here are some of the interesting extentions you much likely want to install:
 Web development
Firebug is a tool you absolutely must have to do web development.
There are two important settings you must change to test websites:
Type about:config in the navigation bar, search for these strings and set both these boolean values to false. You want to see the actual changes done to your website, not some old cached copy.
User Agent Switcher is a nice plugin you likely want if you want to give mobile phone users something different from what normal browsers get.
 Making Firefox usable
 Setting a sensible minimum tab width
It used to be possible to make Firefox tabs shrink as more tabs are opened by going to about:config. The Firefox developers decided that this was too easy and make the tab related options there do nothing.
The current solution is to make a file called userChrome.css in the chrome/ directory in the Firefox profile folder ($HOME/.mozilla/firefox/$profilename).
This will make tabs shrink as more tabs than what is possible with the window with are opened.