STFU && RTFM

When searching for new Add-ons, you are by default shown five results. You might find it more efficient to increase this number.

1. In the address field, go to about:config
2. Accept the warning
3. Search for getAddons
4. Change extension.getAddons.maxResults to 10 (or higher if you want to see even more

When you want to view the source of a page, it opens up in browser popup. However, if you want, you can make it open in your favorite editor, instead of having to cut and paste. To achieve this: do the following:
 

1. In the address field, go to about:config
2. Accept the warning
3. Search for view_source.editor
4. Change view_source.editor.external to true
5. Change view_source.editor.path to the exact path to your editor of choice

In Firefox 3, bookmarks are automatically saved. The only problem is the way they are stored, as they are stored as places.sqlite, as opposed to saving them in HTML. You can change this easily however, by doing the following:
 

1. In the address field, go to about:config
2. Accept the warning
3. Search for autoExportHTML, which should present you with browser.bookmarks.autoExportHTML
4. Change this to true

The “Awesome”-bar, otherwise known as the address bar, has a few nifty little things you can tweak. First off, you can turn it back into a boring old address-bar. To do this you go to the (for now) awesome-bar, then direct your browser to about:config. In the filter enter browser.urlbar.maxRichResults, and change its value to -1.
 
If you only want it to show you websites whose URL you’ve actually entered the address for, go to about:config and find the entry browser.urlbar.matchonlytyped. Change its value to true.
 
To remove addresses from the Awesome-bar that you haven’t visited, you once more go to about:config. Find the entry places.frecency.unvisitedBookmarkBonus, and set its value to 0.
 
Lastly, to remove all addresses from the Awesome-bar, you go to about:config, find the entry places.frecency.bookmarkVisitBonus, and set its value to 0.

There has been so many discussions about where to place the close-buttons for tabs that Mozilla has created a config key for this specific option.
 
To access the key, open a new tab, enter about:config then press enter. In the filter search for browser.tabs.closeButtons. Change this value according to your choice and the following table:
 

I use GMail a lot. I also send a lot of emails, many of those originating from mailto:-links. Hence, it is practical to have mailto:-links open in GMail.
 
This is quickly and simply achieved by going to Tools, Options, Applications. In the search-field, simply enter mailto, then use the Action-field to select GMail, Yahoo Mail or something else, like so:
 

 
Some installations of Firefox do not have the GMail-alternative. This is simply fixed. Open a new tab, then enter about:config in the address-bar. Into the filter enter
gecko.handlerService.AllowRegisterFromDifferentHost, then doubleclick the value so that it is set to true. Then copy/paste the following piece of Javascript to your Addressbar: javascript:window.navigator.registerProtocolHandler("mailto", "https://mail.google.com/mail/?extsrc=mailto&url=%s","Gmail"). When asked whether you want to add GMail as an application, answer “yes”.