Do you have multiple keyboard layouts installed as it is possible that you accidentally switched to another layout ? Note that Windows remembers the keyboard layout per application.

I have multiple keybords installed but this is not question of accidentally switch. My preset language is Slovak and another keyboard do not use for years because with native keyboard can manage all task I need.

Accidental switch of keyboard would be shown also in Chrome and another browsers. But as I wrote the problem is only in FF and lasts about one or two weeks. I have never met with same problem previously.

Does it still happen in Troubleshoot Mode?

Yes proelem eoytiyue also iy troueleshoot mode

Yes problem continue also in troubleshoot mode. I tried to write this sentence when problem appear.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile

to zeroknight I am not sure if I can use Developer edition. On page with download link you sent me is written that this kind of version is not supported on WIN 8.1 and less. My OS is running on WIN7

Did you miss: Note that Windows remembers the keyboard layout per application. If you change the keyboard layout in Firefox then this only applies to Firefox, all other applications will still use the default layout. You would have to make the Windows Language tool bar visible to show the currently selected layout. In the past the combination to rotate the layout would be only pressing a few modifier keys and since Firefox uses almost every shortcut that is available (they usually have a hard time to find a shortcut for a new feature, let alone extensions), you can easily trigger a layout switch.

Despite fact that I have installed two keyboards in win (Slovak, English), I use only Slovak keyboard for years. So this problem is not question of switching of keyboard. It started last week I suppose after install new version of FF. Never had same problem with FF previously.

to zeroknight Yes it happen also in new profile. I made a new profile in FF that is clean. I tested it and problem happens also in new profile.

Go to about:config and try changing accessibility.force_disabled to 1 then restart the browser.

Go to about:third-party and try disabling any third-party modules.

lt asptnnoral (to zerokhnight) Such letter were shown me when I tried to write - to zeroknight.

Change made in about:config and changing accessibility.force_disabled to 1 then restart the browser was not helped. I tried about:third-party and try disabling any third-party modules. There were three 3rd party modules. One of them I stopped. Next two belongs to antivirus. I am not sure about stopping these modules.

Some minutes FF works for me fine as previously. Some minutes it is crazy.

What is your antivirus software? Does it still happen with it disabled?

Can you try a different keyboard ?

to zeroknight ESET IS

to cor-el I tried to switch to ENG keyboard. Problem is same. For example words "alfa beta gama delta" written in FF are shown as "alfa nkta gaja dklta"

I tried to downgrade installation of FF to latest correctly running version but same problem was appear with letters. Meanwhile latest installed FF version is 115.4.0esr

There were three 3rd party modules. One of them I stopped. Next two belongs to antivirus. I am not sure about stopping these modules.

Try disabling the ESET Internet Security third-party modules. If that doesn't work, try disabling ESET itself. Finally, try uninstalling ESET.

to zeroknight

I have installed FF and ESET antivirus for years. And worked together without problems. To uninstall antivirus means to loose protection for all apps in computer and make compute vunerable. There is no problem with ESET product and another web browsers. There is problem with Firefox. This happened about two weeks ago.

Is there any chance how to backup FF with all bookmarks, passwords .. and then fully uninstall FF?

Boot the computer in Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tuto.../how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/

What program(s) did you use to scan for malware as it is possible that it didn't catch it ?

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Temporarily disabling or uninstalling third-party antivirus is entirely safe, Windows has its own protection so you are never completely unprotected. If it works without ESET then you can narrow down your focus to its settings and find a configuration that is compatible with Firefox.

to cor-el for malware detection I use Mawarebytes Free edition. For viruses full version of ESET IS. It was my first feeling about any virus or malware after this problem happen. So I tried to scan computer with another free tools and did not find anything.

Problem I described is shown for seconds ( I suppose less than 1 minute) then FF works OK and then again problem etc. For example this my reaction was written without problem but for some of previous I have to cancel typing and wait for dissapearing of problem.

Do you have any suggestion on software I should use for scanning of probably virus or malware?

to zeroknight OK I can shut down ESET protection and test it. Will inform you accordingly.