Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

I don't think this is the problem -- no errors/warnings appearing in ESET but I can explore further. Shouldn't there be something other than silent failure if execution is blocked? And if it were a Firewall issue, that shouldn't stop the program from launching, right -- just connecting to a website? I have the same version of Firefox installed on other similarly configured laptops (i.e. same security and firewall configuration, same OS) without any problems.

The only Firewall on this box is the Windows 10 firewall. Firefox.exe was set to be enabled in all the relevant rules, but to be safe, I deleted every rule mentioning Firefox and trying to execute the binary again. Still nothing shows on the screen. I also tried installing the latest Firefox Portable -- same result (the Portable splash screen shows but then no web browser window appears). No errors/warnings anywhere. I also checked ESET logs and see no mention of any events involving Firefox.

Isn't there some way to troubleshoot/narrow why Firefox can't open a browser window at all on this system?

When you start Firefox, does it show up in the Task Manager?

If so,

You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox.

Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

firefox.exe (or the portable version) does show up in the task list. Each time I try to launch it, I get another process. But it never shows any browser (or other) window, or other message.

I have completely wiped Firefox from my machine and reinstalled with no effect. By wiping, I mean: uninstalled through the Windows "add and remove programs," followed by deleting every file on my hard drive that falls under "mozilla," followed by deleting every registry key that includes Firefox, followed by rebooting, and then reinstalling. Firefox still won't run.

I have also tried specifically deleting all sessionstore* files without effect.

I can't type about:support<enter> in the address bar, since I don't have any Firefox browser window to type it into.

For what it's worth, I've also tried launching Firefox as administrator, also no luck.

I've also tried firefox.exe -console, and it does open a console window, but nothing appears. (I've also tried every other documented command-line switch, including safe-mode, but nothing makes any difference.)

I called for more help.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

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

Start your Computer in safe mode with network support. Then start Firefox. Try Safe websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

As I noted above, Firefox will not start in safe mode, whether via command-line invocation or shift key. Nothing appears on the screen.

And the computer?

As noted above, it’s a 3rd gen Carbon X1 running fully patched Windows 10/64-bit.

(As a side note, I have a very similarly configured 5h gen Carbon X1 that has no problem with Firefox).

FredMcD said

And the computer?

FredMcD said

Start your Computer in safe mode with network support.

Sorry, I misunderstood your earlier comment "and the computer?". I have rebooted into safe mode with network support and Firefox starts normally. How can we isolate why it's not starting under normal conditions?

Ahh, now we know the problem is not Firefox per se.

As a test, disable your protection programs.

I'm unable to turn off ESET by corporate group policy; however, Firefox works on another Windows 10 computer (the 5th gen Carbon X1) with an identical ESET installation. ESET is also not reporting any issues with Firefox or logging any errors/warnings.

I tried disabling Windows Firewall and it had no effect -- Firefox still wouldn't open.

Sorry for all the trouble -- I figured it out by attaching a debugger to the firefox.exe process. It turns out it was Traps CYV-13009 that was silently blocking the program from launching. A Traps upgrade fixed the issue. https://www.paloaltonetworks.com/documentation/41/endpoint/traps-release-notes/traps-4-1-0-addressed-issues

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.

The clue was the debugger showed the process stuck on cyvera.dll, which apparently belongs to Traps Agent.