With version 29.0, the tab display for Firefox was changed. I and others found it awkward. For me, at least, the contrast is much lower making it hard to see them clearly. I keep many tabs open so it is important for me to be able to see them clearly. Consequently, I reverted to version 28.0.
Unfortunately, Firefox is not the only software that this has happened to where omething that was working fine was arbitrarily changed. My first browser was Netscape. When I found that some pages were not working for me, I tried upgrading to a later version. It was considerably different and was awkward. As a result of that, I ended up switching to Firefox.
I have used Firefox since the early days (version 0.94, I think). Many of the changes over time have been good, but I like the version 28.0 and prior tab display.
Having found something that works, I do not like it being messed up. I switched browsers once because of this. I could do it again.
I think that software developers and others who develop program interfaces (whether GUI or CUI) should consider their existing users. If a change is going to be made, allow an option to choose which interface to use.