Does this happen on all websites or just a specific website?

If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? For instructions on starting in Safe Mode, see Use Troubleshoot Mode in Firefox.

Same problem here. Same version of Firefox

In reply to Chris, which has been delayed because for some reason Firefox has been refusing to Submit my message, this happens on all websites.

I also opened it in Safe Mode, and the problem didn't occur. Back to the regular mode, and the problem still exists. Do I need to change some settings in Safe Mode?

Thanks

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window to disable all extensions. You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

I have the same problem. But if I left-clicked and held the mouse down while typing, the letters don't disappear.

Thanks for the extra advice SRChiP, but that didn't work for me. Will be trying the Safe Mode troubleshooting instructions later this week, but daren't do it now in case I screw up Firefox in the middle of a heavy workload!

By the way, there's a new wrinkle to this problem. After letters in the Find bar start disappearing the Find bar switches itself to Quick Find.