My parameters were "No HTTPS everywhere" and "No DNS over HTTPS (DoH)".

 I cannot acces some website because of their local policy, so I changed my DNS parameters.
 I choose to use DoH with Cloudflare instead of the given DNS.
 Internet worked perfectly.

 I thought it will not load at all (timeout ?).
 Others webrowsers (e.g. : Microsoft Edge) are correctly working.
 The "ping google.com" command is working normaly on windows with 2ms delay.
 The DNS on windows "Get-DnsClientServerAddress" are the normal DNS of the router.
 Even with reverting the DoH parameter to off, I do not get back my web browsing speed.

 It's slow on my home network.
 It's slow on a public network.
 It's slow on another private network.
 https://www.dnsleaktest.com gives me a list of Cloudflare DNS (IPv4) near me, different from the DNS (IPv6) used by Windows 10.
 Meanwhile, Microsoft Edge, ping and wget/curl are working normally (i.e. no delay).
 Noticably, Microsfot Edge use by default according to https://www.dnsleaktest.com the same Cloudflare DNS.

 It's slow on my home network.
 It's slow on a public network.
 It's slow on another private network.
 https://www.dnsleaktest.com gives me one DNS from my ISP (IPv4), same value as the DNS (IPv4) used by Windows 10.
 Meanwhile, Microsoft Edge, ping and wget/curl are working normally (i.e. no delay).
 Noticably, Microsfot Edge use by default according to https://www.dnsleaktest.com the Cloudflare DNS.

 The program oppened succesfully start.duckduckgo.com .
 This website was choosen as it is an allowed website on every DNS.
 Firefox nightly (mozilla-central) 2023-06-02 : good
 Firefox nightly (mozilla-central) 2023-09-03 : good
 Firefox nightly (mozilla-central) 2023-10-20 : good
 Firefox nightly (mozilla-central) 2023-11-12 : good
 Firefox nightly (mozilla-central) 2023-11-24 : good
 Firefox nightly (mozilla-central) 2023-11-30 : good
 Firefox nightly (mozilla-central) 2023-12-03 : good, show a message saying more or less "Nightly started with no extension"
 Firefox nightly (mozilla-central) 2023-12-04 : good
 Firefox nightly (mozilla-central) 775ade8b :   good
 Firefox nightly (autoland) 87d12c4b :          good
 Firefox nightly (autoland) ba8757a7 :          good

 And as of today, it is.

 It worked but I do not understand why!
 * New profile. Using "firefox.exe -profile" I created another clean profile to see.
                It worked.
                According to https://www.dnsleaktest.com, my new empty profile uses both Cloudflare DNS (IPv4) and the router DNS from the ISP.
                Adding one-by-one most of extensions (6 now) to this new profile do not break yet my Firefox speed.
 * Safe Mode. Using "firefox.exe -safe-mode" I have started my corrupt profile without extension, JIT, etc...
              It worked.
              According to https://www.dnsleaktest.com, it uses only the router DNS (IPv4).

Hello fellow Firefox users, I sadly cannot use my Firefox (120.0.1, 64 bits, Windows 10) anymore as it is too slow to show a webpage... It's so slow, that I initially thought that I have no internet connexion at all on Firefox. Can anyone help me figuring why my Firefox profile is so slow ? Long story short: ----------------- - Until 2023-12-05, my Firefox was working normally. My parameters were "No HTTPS everywhere" and "No DNS over HTTPS (DoH)". - On 2023-12-05, I was connected to an entreprise DNS (Mangling DNS). I cannot acces some website because of their local policy, so I changed my DNS parameters. I choose to use DoH with Cloudflare instead of the given DNS. Internet worked perfectly. - On 2023-12-06 and 2023-12-07, same parameters, Internet worked perfectly on another network. - Since 2023-12-08, Internet is very slow. It need about 1.5 minute to get start.duckduckgo.com loading on any network. I thought it will not load at all (timeout ?). Others webrowsers (e.g. : Microsoft Edge) are correctly working. The "ping google.com" command is working normaly on windows with 2ms delay. The DNS on windows "Get-DnsClientServerAddress" are the normal DNS of the router. Even with reverting the DoH parameter to off, I do not get back my web browsing speed. Troubleshooting : ----------------- - It's slow with "DoH and Cloudflare" activated. It's slow on my home network. It's slow on a public network. It's slow on another private network. https://www.dnsleaktest.com gives me a list of Cloudflare DNS (IPv4) near me, different from the DNS (IPv6) used by Windows 10. Meanwhile, Microsoft Edge, ping and wget/curl are working normally (i.e. no delay). Noticably, Microsfot Edge use by default according to https://www.dnsleaktest.com the same Cloudflare DNS. - It's slow with "DoH and Cloudflare" NOT activated. It's slow on my home network. It's slow on a public network. It's slow on another private network. https://www.dnsleaktest.com gives me one DNS from my ISP (IPv4), same value as the DNS (IPv4) used by Windows 10. Meanwhile, Microsoft Edge, ping and wget/curl are working normally (i.e. no delay). Noticably, Microsfot Edge use by default according to https://www.dnsleaktest.com the Cloudflare DNS. - In about:crashes there is not relevent crash (only old crash). - I have not changed any about:config parameters in the 2023-12-05 to today (2023-12-12) period. - I have run mozregression GUI with no problem. The program oppened succesfully start.duckduckgo.com . This website was choosen as it is an allowed website on every DNS. Firefox nightly (mozilla-central) 2023-06-02 : good Firefox nightly (mozilla-central) 2023-09-03 : good Firefox nightly (mozilla-central) 2023-10-20 : good Firefox nightly (mozilla-central) 2023-11-12 : good Firefox nightly (mozilla-central) 2023-11-24 : good Firefox nightly (mozilla-central) 2023-11-30 : good Firefox nightly (mozilla-central) 2023-12-03 : good, show a message saying more or less "Nightly started with no extension" Firefox nightly (mozilla-central) 2023-12-04 : good Firefox nightly (mozilla-central) 775ade8b : good Firefox nightly (autoland) 87d12c4b : good Firefox nightly (autoland) ba8757a7 : good - Firefox 120.0.1 is saying that it is up-to-date. And as of today, it is. - I do have some extensions (12 activated) on my Firefox, so I try without extension. It worked but I do not understand why! * New profile. Using "firefox.exe -profile" I created another clean profile to see. It worked. According to https://www.dnsleaktest.com, my new empty profile uses both Cloudflare DNS (IPv4) and the router DNS from the ISP. Adding one-by-one most of extensions (6 now) to this new profile do not break yet my Firefox speed. * Safe Mode. Using "firefox.exe -safe-mode" I have started my corrupt profile without extension, JIT, etc... It worked. According to https://www.dnsleaktest.com, it uses only the router DNS (IPv4). - I can send some result from about:support if needed, but it kills privacy... Conclusion : ------------ I do not understand why my main Firefox profile is not working correctly. I do not see strange things when troubleshooting. Nothing makes sense. May someone help me figuring out? Regards, eliotlencelot.