> How do I get the old style of tabs back?
Don't know but:
> I need every vertical pixel I can get.
If this is really important to you (as it is for me), try Tree Style Tab (https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...). First just installed it to move the tabs to the side, but now I gotten so used to tabs in a logical tree that anything else is a big hassle to use.