Hi there jeunegeo,Thanks for asking in mozila support.I am Ahmadi Nejad a fellow Firefox user.You asked firefox slow down your network.Please ask to your Internet Service Provider..or if the connection is ok then you can try re-installing your network driver.Or if everything is ok then you can set custom Proxy in Firefox.I know the problem is from your internet,but you could try set a network limit for your browser.Hope this helps!

Regards Ahmadi Nejad

Thanks for your answer, but it's not my internet connection which is bad, but my internal network. My Internet connection works well, only the connection betwen two local computers is affected if firefox is open.

Start Firefox using Troubleshoot(Safe) Mode {web link}

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

Yes, the problem is still here, and I've always try that.

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

Starting Any Computer In Safe Mode (web link) Free Online Encyclopedia

https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/ +++++++++++++++++++++++++++++++++++++++++++++++ If the problem goes away with the computer in Safe Mode, the problem is with one of the other programs on the system.

As a test, disable your protection programs.

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

Starting Any Computer In Safe Mode (web link) Free Online Encyclopedia

https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/ +++++++++++++++++++++++++++++++++++++++++++++++ If the problem goes away with the computer in Safe Mode, the problem is with one of the other programs on the system.

As a test, disable your protection programs.

Hello! Same problem with Windows in safe mode, with or without antivirus. And why only with Firefox? Does Firefox change some network controller settings ?

I called for more help.

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.

thank you for trying to help me. I've always scanned my computer with a lot of antivirus or antimalware. Nothing found!

I specify my question : why I can't read, with VLC, a video stored on another local computer when Firefox is open, when it works well if Firefox is closed? Both computers are on the same network behind a 1 Gb/s router when other browsers (Chrome, Safari or IE) are open, it's ok! This is something specific to Firefox... very strange!

I am now looking at the drivers of my Ethernet card, because it's the only thing I haven't checked.

I'll test and come back.

Hello,

after a lot of tests, I've changed the ethernet-card driver (Realtek), and now it's ok! But I don't understand why?

The problem happened after an update of Firefox, a few months ago

If someone knows, I'm interested ...

That was very good work. Well Done.

As to the ethernet card, I suggest creating a new question.