What's your computer system and Firefox version?

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}

You can check for issues caused by a corrupted or incomplete Visual C++ installation that is missing runtime components (Redistributable Packages) required by Firefox.

See "Visual Studio 2015, 2017 and 2019":

• https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads

What's your computer system and Firefox version?

Thanks for the swift responses. I am running Firefox version 85.0.2 on an Intel I7, 32GB RAM, Windows 10 Pro (VersionVersion - 10.0.19041 Build 19041).

I have removed Firefox from my program's list of trusted or recognized programs, as suggested however to no avail.

Start Firefox in Safe Mode {web link}

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 Secure websites. Is the problem still there?

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

Apologies for the delayed response.

Attempting to start Firefox in SafeMode experiences the same issue, whereby I can briefly see the modal/dialogue flash up before closing down again. Starting my device in SafeMode also has not resolved the problem.

I did note that running one of my web applications in Visual Studio 2019 to be viewed in Firefox continually ran my project even though no browser tab opened. This might not be of much use but thought it was worth a mention.

Do a clean reinstall of the current Firefox release and delete the Firefox program folder before installing a fresh copy of the current Firefox release.

• download the Firefox installer and save the file to the desktop
  *https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

If possible uninstall your current Firefox version to cleanup the Windows Registry and settings in security software.

• do NOT remove "personal data" when you uninstall the current Firefox version, because this will remove all profile folders and you lose personal data like bookmarks and passwords including personal data in profiles created by other Firefox versions

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (64-bit Firefox) "C:\Program Files\Mozilla Firefox\"
• (32-bit Firefox) "C:\Program Files (x86)\Mozilla Firefox\"

• it is important to delete the Firefox program folder to remove all the files and make sure there are no problems with files that were leftover after uninstalling

Your personal data like bookmarks is stored in the Firefox profile folder, so you won't lose personal data when you reinstall or update Firefox, but make sure NOT to remove personal data when you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data.

If you keep having problems then you can create a new profile as a quick test to see if your current profile is causing the problem.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Reinstall Firefox

• https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems

Thanks for the response.

As suggested I have complete a full fresh install of Firefox, via the link provided ensure that the directory in (64-bit Firefox) "C:\Program Files\Mozilla Firefox\" had been deleted. Again the problem remains and I have already tried to create a new profile as stated in an earlier post. Nevertheless, I attempted to create a new profile, however as I cannot access the browser, I took the approach to run Windows + R to open to Run prompt. From here I executed the firefox.exe -P command and as before this briefly loaded a dialogue before closing down again.

Thanks for the suggestions anyway. All good help.

You did not say if you tried to

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there? http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

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

My apologies I missed this out. I have started my device in safe mode and have been able to access Firefox. The browser loads correctly and is all operational, however loading Windows normally results in the error occurring.

Good. Now we are getting somewhere.

As a test, disable your protection programs. Then start Firefox.

Disabled all protection programs as suggested by no luck. Thanks.

Then it's something else on your system.

In continuation of this thread, I have made some advances. Over a week ago I completely installed Firefox and disregard the issue for the time being as other browsers are available to use. Yesterday I had some time a reinstalled Firefox and to my surprise loaded and ran smoothly. Therefore I imported my bookmarks etc and set up the settings as needed and late last night stopped work for the day with everything working as expected.

Note that I have made no changes since in between uninstalling and reinstalling the application. However, this morning opening the browser has once again returned to the previous state of play, where it briefly loads and then closes again and therefore is once again unusable.

Any thoughts?

Were you previously running other software that may have loaded libraries ?

You can check for issues caused by a corrupted or incomplete Visual C++ installation that is missing runtime components (Redistributable Packages) required by Firefox, especially VCRUNTIME140_1.dll.

See "Visual Studio 2015, 2017 and 2019":

• https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads

Thanks, cor-el.

Completed the install and restarted my device and loaded firefox. This opened a dialogue stating that the browser closed with an unexpected error with options start in Safe Mode or Restart the application.

This dialogue was present until it I loaded the project I have been working on in VS2019, which then resulted in the repeated issue occurring of the browser opening and then closing pretty much instantaneously.

Can't be sure that VS cause the dialogue to close however until then I had more access to Firefox than previously.

Thanks.

If it helps I can see a VCRUNTIME140.dll file in C:\Program Files\Mozilla Firefox. Also worth noting that I had both Firefox and VS2019 running simultaneously and working as expected yesterday. Thinking back to the first time I encountered the issue, I opened Firefox as a fresh new install and used it for the rest of the day. Upon shutting down my device and then attempting to the use browser the next day, I was then faced with the same problem as now. It would seem that the browser would work on the day of install but thereafter I would experience the issue explained here and in the original post.

I know this has been silent for a while, but I still have no resolution to the problem I face with Firefox. I have made some progress and have been able to access Firefox whilst my device is in SafeMode. Through further research, I found a similar thread that suggested changing the name of the Firefox executable to something like Firefox1. In my progress for a solution I have tried this and to my surprise, this works. Renaming Firefox.exe to Firefox1.exe will load the browser perfectly every time, however I don't want to particularly fiddle with executables to get the browser working. I have since renamed it back to the original Firefox.exe and again I experience the same issue. I did note somewhere that if the works whilst renamed it is down to malware but I have run endless Windows Defence scans will no issue found.

Any suggestions or advice?

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed free to use malware scanners. Each works differently. If one program misses something, another may pick it up.