In Windows itself, I’ve set the keyboard focus rectangle and keyboard cursor to be thicker than normal. (Control Panel > Ease of Access Center > Make the computer easier to see, under Make things on the screen easier to see.) However, those settings don’t translate to Firefox. To fix this:
Go to about:config. Ignore the warning if you get one.
Right click on the screen and choose New, then Integer.
Call it ui.caretWidth and enter your chosen value. You can modify this later if you need to.