Firefox 4/5: How to change the minimum tab width
In firefox 2 and 3 it was possible to set the browser.tabs.tabMinWidth within about:config to zero so that tabs shrank in width the more tabs you had open until just the icon was left. In Firefox 4, that option has been removed, leaving you with fixed width tabs of 140 pixels.
Thankfully, although the option has been removed, the functionallity has just moved. In firefox 4 and 5 you can customise the Chrome of the browser by using CSS styles in the userChrome.css file.
This file is stored within your firefox profile. To get to that directory, click on the Help menu and then select Troubleshooting information.
On the tab that opens you’ll see that the first section is Application Basics and in that section is Profile directory with a button next to it saying Open containing folder. Click on the button.
Open the Chrome folder. In there you’ll find two example files, one of which is userChromeExample.css. Copy that file, renaming it to userChrome.css and paste in the following CSS style:
min-width: 0px !important;
max-width: 140px !important;
Save the file and restart the browser. Now you should have tabs that behave like you’re used to in Firefox 3.