Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

Can you make the Password Manager window wider?

You can resize various pop-up windows like you get for cookies and passwords if you place the mouse pointer in the bottom right corner. To be able to fully expand this window you can temporarily switch to Full Screen mode and drag the border to make the window as large as possible. Look for a close X in the top right corner or use Ctrl+W if you can't find the close or confirm button.