Do you mean the tabs on Firefox's tab bar? Obviously the active tab has rounded corners, and when you hover a tab, that tab has rounded corners, but the others do not have any corners whatsoever.

That personally annoyed me when I upgraded to Firefox 29 so I created a custom style rule to address it: https://userstyles.org/styles/100627/outline-background-tabs-firefox-29-australis (old screen shot attached) Then in response to requests, I created some additional ones. Those are linked in the comments on that page.

I have no idea whether those old style rules work or look decent on Windows 10. If you test, please let me know.

Of course, there may be other add-ons on the Add-ons site for this, but I haven't researched it.

Many of us are using; Classic Theme Restorer (Customize Australis) {web link} Restore squared tabs, appmenu, add-ons bar, small button view and more on Australis UI (Windows/MacOSX/Linux + Fx 29+).

This will restore many of the things the last upgrades took away.