Oh, that is very broken. Recent versions of Firefox display a small blue icon on the right side of the player for picture-in-picture. Something seems to be wrong with that icon on yours. If you don't use this feature, you could disable it here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste videocon and pause while the list is filtered

(3) Double-click the media.videocontrols.picture-in-picture.enabled preference to switch the value from true to false

Any improvement?

More info on about:config: Configuration Editor for Firefox.

Alternately, if you want to get that feature working, I would lean toward replacing the program files, but perhaps someone else has an easier idea. (Before doing anything drastic, I suggest backing up your data: Back up and restore information in Firefox profiles)

What Firefox locale do you use in case your current locale is broken ?

• Options/Preferences -> General -> Language:
  Choose the languages used to display menus, messages, and notifications from Firefox.

Are you using a language pack that provides string translations for the user interface?

• https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack ?

You can try to reinstall Firefox on top of the current installation.

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release