What kind of repair are you talking about?

It is possible that your firewall or other security software blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full, unrestricted, access to install for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/server-not-found-connection-problem
• https://support.mozilla.org/kb/Firewalls

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Sorry for my translation for "réparer" ; I think that in English the correct word is "refresh"; anyway, it is the resetting of Firefox to the factory parameters. Once I do that, Firefox works 100% normally. I tried to re-introduce Firefox as exception in the Windows firewall, but the results were the same. I tried then to start Windows in safe mode. The result was the same, except that I got an error message : "Could not locate the address www.xxxxx on the server", for all Internet sites. One point to mention is that this problem appeared just after the Firefox update to version 51.0.1.

hi, this sounds like a problem that may be caused by the new multi-process architecture in firefox (codename "e10s"): https://blog.mozilla.org/futurereleases/2016/08/02/whats-next-for-multi-process-firefox/

please try disabling e10s like this: enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named browser.tabs.remote.autostart.2. double-click it and change its value to false and restart the browser.

Thank you very much ! Without this forum, I would never have found the answer to my problem !