Why Mozilla Firefox 71 Might Fail to Start on Windows

Mozilla rolled out a new version of Firefox earlier this week for all supported desktop platforms, but according to a number of reports, not everyone is getting a completely flawless experience.

And by the looks of things, the problems that some users experience aren’t necessarily related to the improvements that Mozilla introduced in the browser, but to the way the application interacts with the security software running on a device.

More specifically, some users complain that Mozilla Firefox no longer runs on their device after updating to version 71. In other words, a perfectly working instance of Firefox 70 is updated to version 71 using the built-in update engine, after which the browser no longer launches.

This is something that several users have reported and which Mozilla is apparently aware of, as a discussion on Bugzilla confirms the company noticed the complaints posted this week by users who updated the browser to version 71.

As per TechDows, the issue was first spotted nearly two months ago, and at that time it affected Windows 8. However, the Firefox 71 problem exists on Windows 10 as well, but there’s a good chance this is strongly related to the security apps that are installed on the device.

The first thing to check is the antivirus application that protects the computer, other than Windows Defender – at this point, there don’t seem to be any issues with Windows Defender interfering with Firefox 71, and no reports of such issues have been received. On my device, Firefox 71 also seems to be working correctly with Windows Defender up and running.

According to more recent posts on Bugzilla, Comodo software is most likely the culprit for issues with Firefox 71 – as a matter of fact, Mozilla is also planning to add this as a known bug in the release notes of the latest browser version.

So the first thing do if you are indeed running Comodo products, such as Comodo Firewall, on the device is to make sure they are updated to the most recent release. If you’re already running the newest version of Comodo software and still hitting the issues, the only workaround that you can try is to disable or remove it completely.

As posted on Bugzilla, some people don’t even know they were running Comodo. In some cases, software developed by this company was already removed from the impacted device where Firefox 71 failed to launch, so the workaround in this case is a little bit different.

In this case, users need to manually remove one DLL file that is injected into the Firefox process by Comodo software and which for some reason remains on the device after removing the security applications. The file is called:


And is typically located in:


After you delete this DLL file, make sure that you reboot the device to apply the changes.

There are users, however, who claim that Comodo wasn’t and isn’t installed on their devices and Firefox 71 still fails to launch. Similar issues could be encountered with any other security product on the device, and the easiest way to check if there’s a compatibility bug breaking down the browser is to boot into safe mode without any services loading at startup. If Firefox 71 launches correctly in Safe Mode, then checking the security software on the device is a good place to start the diagnosis.

Mozilla is still looking into the issue, and I expect it to be fixed shortly by Comodo. Additional optimizations are also expected in the first point release for Firefox due by the end of the month.

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