> No matter how many tabs I have open they remain usable size
They were! I think they've shrunk, though that might be an optical illusion... And I can't see a simple way to make them bigger (simple as in: not editing the css file).
There is a pref that controls the minimum width now (browser.tabs.tabMinWidth). The default changed from 100px to 76px in Firefox 57. You should be able to tweak this to change the minimum width.
They were! I think they've shrunk, though that might be an optical illusion... And I can't see a simple way to make them bigger (simple as in: not editing the css file).