I use Linux as well. Are you using a theme?

Released an updated version. It should now work in Firefox 70. Let me know whether or not it does. Sorry for the delay.

Technically, this is what Mozilla calls a dynamic theme. It is clearly stated both on the Preferences and the Usage page (link to the Usage page is in the Preferences page) the problem with using this addon and themes at the same time.

Actually, it can work even without enabling the titlebar. The layout is different, since the window min/max/close buttons are moved within the actual window in the upper right, since there's no titlebar. To handle this, there is a setting in the Preferences for Titlebar enabled or disabled. Again, this is covered in the Usage page, along with screenshots.

There actually is no switching profiles, but rather it starts another instance with the desired profile, which you can get to quickly by opening a new tab and entering "about:profiles" into the address bar.

Perhaps you should check the Preferences setting for the Titlebar enabled/disabled setting. There is a lot of info about configuring the spaces in the Usage page (link to the Usage page from the Preferences page).

I think it might be possible to set font and perhaps color. You might think the outline is untidy, but I found it made it easier to see because it sets it off, but I could also make it optional. I've actually given this some thought, probably using the sidebar for the dialog, so one can easily see the changes. However, unless a lot of people start using this, it's hardly worth the effort.

I primarily made this because I personally wanted it, couldn't find one that worked well, and thought others might find it useful. It's definitely more about function than looks.

I'm not sure that an avatar (like Chrome?) is feasible. I'm definitely not interested in such eyecandy. :P