As you open more and more tabs in Firefox it becomes more difficult to identify the one you are looking for at the reduced tab width. But if you are willing to sacrifice a few pixels (eight to be precise), you can get an extra line for tab titles with ModestTab, effectively increasing tab readability. Slightly scaled up site icons also help to identify a tab more quickly.
It also helps make Firefox look more roomy and relaxed.
ModesTab is available for download from Mozilla Add-ons. Despite the warning that says it is for Windows only, it works perfectly on Linux at least. Usually this kind of message mean the developer hasn’t tested it on all platforms.