> Before Firefox 3.6 (probably that version), Firefox was my most used browser, but after that version, Firefox started getting slower and more buggy.

Haha, I remember that same feeling, with 3.6 being "peak" Firefox back in the day. My 3.6 was heavily hand-tailored to my needs via about:config etc. Just some dedicated end-user here, but I did know it very well. Version 4 felt considerably worse on a WinXP system, some essential-to-me add-ons broke, etc. I remember feeling really - as in, really - frustrated when I finally had to make the switch.

Apparently, 3.6 is the longest supported Firefox version ever, 27 months: https://en.wikipedia.org/wiki/Firefox_3.6#End_of_life