Unknown exception is hard to troubleshoot...

To rule out corrupted program files, could you:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. It's not essential to uninstall Firefox, but you can if you like, saying No to any request about removing personal data.

It only takes a few minutes.

(A) Download a fresh installer for Firefox to a convenient location. You can choose between:

Regular version for Windows 7-10 https://www.mozilla.org/firefox/all/ Note: only runs Flash, no other legacy plugins "Windows" version is standard 32-bit version; to take advantage of more memory, you could switch to the 64-bit version, see: How to switch from 32-bit to 64-bit Firefox

Extended Support Release for Windows XP-10 https://www.mozilla.org/firefox/organizations/all/ Note: Windows (32-bit) runs legacy plugins that ran in Firefox 51; 64-bit runs Flash and Silverlight. If anyone reading this is thinking of switching to the 64-bit version, see: How to switch from 32-bit to 64-bit Firefox

(B) Exit out of Firefox (if applicable).

(C) Using Windows Explorer/My Computer, rename the program folder as follows (you might have one or both):

C:\Program Files (x86)\Mozilla Firefox =to= C:\Program Files (x86)\OldFirefox

C:\Program Files\Mozilla Firefox =to= C:\Program Files\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Any improvement?

Dear jscher2000, thank you for the instructions! I will try them in time of 12 hours and report here.

Does the Windows Event viewer shows more detail about the crashing module?

Did you try both the 32-bit and 64-bit Firefox version?

You can find the full version of the current Firefox release (56.0.2) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/
• https://www.mozilla.org/en-US/firefox/56.0.2/releasenotes/

Dear cor-el, thank you for answer! As far as I remember Windows Event Viewer had not provided any useful info, it showed like there was window with text that I already provided in 1st post, but I'll check again!

I will check Firefox versions and try, currently I'm not even sure which one, 32 or 64 bit, I using!

I will follow jscher2000 and your tips today (UTC+03:00)!

Dear jscher2000 and cor-el. I finally got time to answer. The problem of my client was solved just in the same day you answered me, in morning! So, client tried by himself reinstall and all these things I described in my top post in the topic. jscher2000, thanks for instructions, I will follow them in future to resolve such kind of problems, they are very helpful!

And advice of cor-el finally helped! Client installed 32-bit version of Firefox and this solved the problem, he says he was shocked about easiness of solution :) !