I somehow lost the find bar and cannot find out how to open it again. I really need it to search the text in documents. Please help.
It is possible that the screen is too high and that the scroll bar and the find bar and add-on bar fall off at the bottom.
If that works then first close all other open Firefox windows and then close Firefox via "File > Exit/Quit" to save that setting.
If the above didn't help then see:
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).