Tried reinstalling, disabling antivirus, Chrome works fine? Sounds like Firefox itself isn't the culprit.

Here's what to try:

Clear cache and cookies in Firefox settings. Sometimes corrupted data can cause connection issues. Check your network settings. Try disabling any proxies or VPNs you're using. Also, try connecting to a different network (e.g., mobile data) if possible. Reset Firefox profile. This resets settings to default, but keeps bookmarks and history.

Hello Khan Jani, I have already uninstalled and reinstalled Firefox. Of course I have cleared cache and cookies. Also, I have disabled antivirus. I am not using any proxies or VPN.

So, nothing happened as Firefox still not working

Also, I disabled Kaspersky, but issue hasn't been resolved.

Did you try disabling "Inject script into web traffic to interact with web pages"?

Disabling Kaspersky might not be enough, try rebooting in Windows Safe Mode.

I disabled "Inject script into web traffic to interact with web pages", but nothing happened.

Try changing DNS over HTTPS (DoH) to "Off" or "Max Protection" and in Connection Settings change "Use system proxy settings" to "No proxy".

Hello zeroknight,

I tried your suggestions, but nothing happened. It seems like my Firefox browser cannot connect to Firefox servers.

Also, when clicking "About Firefox", then it comes "Firefox failed to check for updates".

Try this. https://forum.kaspersky.com/topic/getting-pr_connect_reset_error-in-firefox-when-using-kaspersky-total-security-28248/

Hello jonzn4SUSE,

I have see already your proposed link. So, this user had a problem to open any Tab by Firefox, when Kaspersky was on. When Kaspersky off he had no problem. I have already fully disabled Kaspersky, but the issue is not resolved. Could this issue be done because of the recent updates of Windows 10 ? Firefox could resolve it together with Microsoft ?

I don't really understand what happened with Firefox. So, Firefox was always very slow to reach a website at the start when opening my device (while other browsers are much faster). This may caused a corruption to a file in my registry when today after start of my PC, I opened Firefox and then Firefox tried to reach the website I have by default. So, Firefox might be too soft for such work to open a website when I started my computer, as last time websites are becoming even stronger and stronger.

Then, I uninstalled Firefox, reinstalled again, deleted cache, cookies, history, everything.... On the other hand, Windows are keeping some files in their registry as they do not delete everything when uninstalling a program, for example Firefox. Thus, if such corrupted file has been left in the registry of my Windows 10, when reinstalling Firefox the issue was not resolved and Firefox was still not working.

Can the management of Firefox advice anything for such issue to be resolved ?

I have already fully disabled Kaspersky

You would need to uninstall Kaspersky to fully disable it.

So, Firefox was always very slow to reach a website at the start when opening my device

Kaspersky can cause performance issues if you have "Inject script into web traffic to interact with web pages" enabled and don't have the companion browser extension installed.

Do you still see PR_CONNECT_RESET_ERROR with Developer Edition?

Hello jonzn4SUSE, I have see already your proposed link. So, this user had a problem to open any Tab by Firefox, when Kaspersky was on. When Kaspersky off he had no problem. I have already fully disabled Kaspersky, but the issue is not resolved. Could this issue be done because of the recent updates of Windows 10 ? Firefox could resolve it together with Microsoft ?

If you're running the pro version of Windows, see if you have the same issue Windows Sandbox. This will show you that it's a Kaspersky issue. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

I would also suggest you post your issue in Kaspersky forum and that guy will reply.

Well I have uninstalled Firefox for many times and then reinstalled again, but nothing happened. I changed Profile and deleted the old profile. Deleted cache, cookies, history, everything. The Firefox looked brandnew Also, I paused antivirus, but nothing happened again.

So, I think for an unexpected reason (maybe strong and too long Windows 10 updates, or Firefox update, or ??) a file of Firefox was broken. Thus, by uninstall not all files of Firefox are deleted. There are dozens of files of Firefox kept in several positions in the registry of Windows 10 and these are not deleted by uninstall. I believe that even the software engineers of Microsoft do not know the exact positions of the dozens of files kept in the registry, after uninstalling a program.

Thus, this issue could be only resolved by a new version of Firefox, in which the software engineers of Firefox would apply a method to overhaul the old broken files of Firefox in the registry.

Also, I paused antivirus, but nothing happened again.

Pausing is not enough, try uninstalling Kaspersky or rebooting in Windows Safe Mode with networking enabled.

I came in contact with the people of Kaspersky and they absolutely assured me that if I pause their program is exactly the same as to uninstall it. So, by pausing Kaspersky is exactly the same that it is not installed in the computer. During recent updates of Windows 10, Firefox crashed and I got two times notices to send information about crash to Firefox. There are two ways such issue to be resolved : Either Firefox to prepare a new version to overhaul the possible corrupted files in the registry, or Microsoft in their next updates of Windows 10 to take care when uninstalling a program everything to be deleted. For what reason they leave dozens of files in the registry after uninstalling a program ? This is not normal and should be changed.

Pausing an application is not equivalent to uninstalling it for troubleshooting purposes. Software bugs do not care about developer assurances.

https://support.kaspersky.com/KESWin/11/en-us/128006.htm

Pausing and resuming computer protection and control Pausing computer protection and control means disabling all protection and control components of Kaspersky Endpoint Security for some time.

The application status is displayed using the application icon in the taskbar notification area.

If you're running the pro version of Windows, see if you have the same issue Windows Sandbox. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

I checked it by the traditional way of tree/f in cmd, but didn't find any problem.

Firefox can't load websites but other browsers can

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

I followed all the suggestions, but nothing happened.

Are there any active policies listed in about:policies?

Try changing security.enterprise_roots.enabled to false in about:config then restart the browser.

Does it still happen in Developer Edition?