Hi Howard, is the problem on your Windows 7 with the Extended Support Release of Firefox 115, or on a Windows 11 computer with a different version of Firefox?

The message usually indicates that a file is corrupted/non-matching in the program folder (C:\Program Files\Mozilla Firefox\). If Firefox is launching from the same program folder every time, it wouldn't make sense to have the message intermittently. Is it possible that there is a firefox.exe installed in more than one place on the system? If doing a global search, make sure Windows is set to show hidden files and folders.

This report of another Windows 11 user is troubling: https://www.reddit.com/r/firefox/comments/1k17niv/couldnt_load_xpcom_error_persists_through_fresh/

My apologies for the slow response. I am retired and only check emails after lunch.

The problem is with Firefox is only on the Windows 7 computer which runs Version 115.24.0esr (64). The Windows 11 computer has not had an issue. Foxfire on that machine was version 138.0.4(64) when I first checked today but was downloading and has now installed version 139.0.4(64).

I did a check of the Win 7 C:\ drive and found two versions of Firefox.exe -

    one under C:\Program Files\Mozilla Firefox - used by a desktop icon
    one under C:\Program Files (x86)\Mozilla Firefox\ Firefox.exe - used by a quick Launch Toolbar icon

On this computer I normally normally used the toolbar to load Firefox but I sometime load Firefox indirectly via the Roboform password manager.

I did not see any file named XPCOM*.* in either directory tree.

I was unclear about when the warning message appears. It is not intermittent - it always appears. However it is not always immediate upon opening. Some times, particularly when entering from Roboform, it will appear after I am already in the app that I had launched.

If you go to those folders and launch firefox.exe directly from there, does one of them have the error and the other not? I wonder whether they are even the same version.

Checked both Firefox.exe files - same size different dates. Looked for "115" in a file viewer with results below. Did a byte-wise compare which claimed they were identical.

Started the version in Program Files and got an immediate warning message. Clicked OK and the message closed without launching.

Started the version in Program Files (x86), launched Yahoo Email, opened and read a couple files (about 10 min), closed Firefox without closing email first, and THEN got the warning message.

Relaunched Firefox and email to send this reply. No complaint so far.

Howard

C:\Program Files\Mozilla Firefox\firefox.exe 684,608 .a.. 5-19-25 13:13:23

00058014 hg.mozilla.org/releases/mozilla-esr115/rev/3f9b207bc3d6654d18779a89c 00058058 b2d56e1ab1e2f83•VirtualAlloc2•https://crash-reports.mozilla.com/subm 0005809C it?id={ec8030f7-c20a-464f-9b0e-13a3a9e97384}&version=115.24.0&buildi 000580E0 d=20250519144442•MOZ_APP_SILENT_START=1•MOZ_APP_ALLOW_WINDOWLESS=1•M 00058124 OZ_DISABLE_POISON_IO_INTERPOSER=1• 000000000000•00•115.24.0•Failed t

C:\Program Files (x86)\Mozilla Firefox\firefox.exe 684,608 .a.. 5-27-25 23:06:15

00058014 hg.mozilla.org/releases/mozilla-esr115/rev/3f9b207bc3d6654d18779a89c 00058058 b2d56e1ab1e2f83•VirtualAlloc2•https://crash-reports.mozilla.com/subm 0005809C it?id={ec8030f7-c20a-464f-9b0e-13a3a9e97384}&version=115.24.0&buildi 000580E0 d=20250519144442•MOZ_APP_SILENT_START=1•MOZ_APP_ALLOW_WINDOWLESS=1•M 00058124 OZ_DISABLE_POISON_IO_INTERPOSER=1• 000000000000•00•115.24.0•Failed t

Thanks for checking that. So it sounds like the one in Program Files without (x86) can be removed, but to test that, try renaming the folder to

C:\Program Files\Mozilla FirefoxOLD\

And then testing a link in Office to make sure it opens in Firefox and doesn't panic because now it can't find your default browser.