"Lu Wei" wrote

| I am using Serpent 52, descendant of Firefox 52, tried both "safe mode"
| and with extensions. Tried user-agent switching but not work. No
| anti-virus or other security software installed.
|
| Well, when I was about to install Chrome49, it opened normally. So it
| surely was network problem. Maybe dns or proxy related.
|

It just opened fine for me and I don't allow script,
cookies, 3rd-party images, etc. So maybe it was temporary.
I think worrying about extensions and browser version is usually
what, in the US, we call a "wild goose chase". (A lot of work
for nothing.) It's the first advice many people give, but they're
not the ones who have to waste time with safe mode and
trying other browsers.

There are some sites these days that act up if you don't have
a new browser. I find that setting the userAgent to Windows 7
with a recent version of Firefox seems to work fine with those.
Note that with recent Mozilla products they tried to break that.
It only works if you also add the setting:

general.useragent.enable_overrides true

Then add something like:

general.useragent.override

Mozilla/5.0 (Windows NT 6.1; WOW64; rv:79.0) Gecko/20100101 Firefox/79.0

Firefox changes so fast it might be easier to just set the version
to 200. If they test for something like " 66" then 200 would work
fine. But I haven't tried that. I just update it every once in awhile.

Note about XP: New Moon is a variant of Pale Moon, which
is a less bloated version of Firefox. You can get a recent
release he

https://o.rths.ml/palemoon/?sort=date&order=desc

I currently have 28.10.2a1, about 1 month old. It also has
some nice extras. For example, it incorporated the staus-4-eva
status bar extension as part of the progran and now provides
preferences for the status bar. All my older extensions also
seem to work with it.