Thank you for the detailed analysis. Could you try ruling out any problems in Firefox's program folder as follows:

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, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 40.0.2 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

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

(C) Rename the program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Will Firefox start after a reinstall?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

If fresh program files do not help, could you test a clean profile? This assumes you can get the profile manager to run:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

This will be a parallel folder and will not disturb your regular profile.

Exit Firefox and start up in the Profile Manager using Start > search box (or Run):

firefox.exe -P

Don't delete anything here!

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, assign a name like TEST081815, and skip the option to relocate the profile folder. After creating the profile, select it and start Firefox in that profile.

Will Firefox run?

When returning to the Profile Manager, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

Actually, it seems you already did those things but I missed that paragraph the first time. Apologies for that.

I have not noticed any issues in an environment using Trend Micro Worry Free Small Business Security, but it might have slightly different features than OfficeScan.

There is another thread going with a similar "only works when run as administrator or in Windows Safe Mode": https://support.mozilla.org/questions/1078544

One alternative to consider is the Extended Support Release (ESR) version of Firefox? It is based on Firefox 38 but receives security patches at similar intervals to the standard release. Enterprise software vendors might be using this version for testing (I assume they test...).

You can read more about ESR here: http://www.mozilla.org/firefox/organizations/

To install it, I suggest:

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, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 38.2.0esr from https://www.mozilla.org/firefox/organizations/all/ to a convenient location. (Scroll down to your preferred language.)

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

(C) Rename the program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\Fx40

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\Fx40

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

Does that install and run?

Note: Some plugins may exist only in that old folder. If something essential is missing, look in these folders:

• \Fx40\Plugins
• \Fx40\browser\plugins

Hi jscher2000,

Thank you for your help. ESR version did solve the issue. Now I just have to hope that by the time the 40-ish ESR version comes out the issue will have been solved somehow. Using the ESR doesn't feel like a long term solution if the issue comes from the feature set of the 40.0.2 version.

I've looked into the question you suggested as a similar, and there is a significant difference: with anti-virus off, my FIrefox started. I feel that the issue really is with OfficeScan, but without much more conclusive evidence I have the feeling that the IT services of my company won't escalate to TrendMicro. Still, using an ESR might solve for her/him too.

For now I'll go on with ESR, and if the issue ever comes back I'll see what can be done then.

(Edit: Fixed typo)