Thought I’d share a fix for something that has been bothering me lately. That being the darn empty space in my Firefox tab bar which is there only when the browser is not maximized. So here are some before and after pictures.
before
0_1541073473385_firefoxwithtab.png
after
0_1541073511528_firefoxafter.png

See, much better. So now to the fix.
First -> in the search bar go to your profile

about:profiles

Then select the root directory

alt text

Then navigate to the chrome folder.
In the chrome folder you will either need to edit or create

userChrome.css

Paste this line of CSS into the file

.titlebar-placeholder[type="pre-tabs"] { display:none !important; }

Save the file. Restart Firefox. No more nasty empty box for you! :)