A lot of people had a problem (and some still have) with Firefox 4+'s interface. "Chrome knockoff" and "dumbing down" are two terms that you might see pop out in internet discussions about Firefox's current interface. While I think the current UI is not that bad because it saves some more space and thanks to its minimalist style lets you focus on the job at hand, I understand and sympathize with people wanting to turn back the hands of time and return to the good old trusty Firefox 3.6.
If you use a lot of tabs or if you do a lot of stuff at the same time, sooner or later you'll feel that the tab bar is not big enough. Luckily, Firefox has a great way to organize your open tabs called Tab Groups (also known as Panorama). We've made a video tutorial to teach you all you need to know about it.