Your User Agent shows: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:92.0) Gecko/20100101 Firefox/92.0/eSv19J8f-50

Note the junk at the end.

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

This could be a problem with Avast Anti-Track, so if you use this software then try to disable it.

Hi guys, The problem has gone in Safe Mode. I am using Avast Anti-Track and I'd like to keep doing so.

If it works in Troubleshoot Mode and in normal mode with all extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) disabled then try to find which extension is causing the problem by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via one of these:

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

It's a clean install, I don't have any extensions.

It could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

No, still got the problem after disabling Hardware Acceleration

This is likely caused by Avast Anti-Track modifying your user agent and adding some random characters and you will have to contact Avast about this if that causes problem for you. We can't do anything about this apart from identifying Avast Anti-Track as a possible cause.

OK, thanks for your efforts. At least I know the cause now