MX19.4 after recent upgrade fails to open the browser window but four processes associated with firefox are running. ps -e | grep firefox results in the following in ascending order of process number, firefox, firefox-bin, firefox-bin, firefox-bin. kill the highest firefox-bin process and the browser opens.
Did a complete re-install of MX19.4 and firefox opens. Then did an upgrade and the same problem occurs with firefox.
The firefox which fails is 105.....
Successfully installed MX21.2.1 with upgrades on another drive and no problem with firefox

There is nothing fancy about my hardware. Use stock std modules that come with the distro
00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Kaveri [Radeon R5 Graphics]-Processors-
AMD A6-7400K Radeon R5, 6 Compute Cores 2C+4G 0 0:0 3500.00 MHz

Try opening Firefox from the command line. It may well throw some helpful error messages to the terminal. If it does, post them here, being sure to surround any terminal output with "code" tags, which become available when you click the "Go Advanced" button beneath the compose post window.

Thanks for this test option. Opened firefox on cli, not in background.
after killing last firefox-bin process and firefox opens
[GFX1-]: glxtest: VA-API test failed: process crashed. Please check your VA-API drivers.

closing/exiting (don't recall if single open tab closed or whole window closed)
[2022-09-27T19:14:18Z ERROR viaduct::backend::ffi] Missing HTTP status
[2022-09-27T19:14:18Z ERROR viaduct::backend::ffi] Missing HTTP status
Sandbox: Unexpected EOF, op 0 flags 00 path /proc/cpuinfo

An additional test i did. Transferred an up to date working MX19.4 system including firefox from another PC to the cleaned partition on this my reserve PC where I'm undertaking the firefox tests. This reserve PC has a few hdds/ssds and partitions where I can carry out tests. All baremetal, no VMs. Still the same problem.
I thought at one stage it was hardware related but installing MX21.2.1 and getting firefox to function properly negated this thought.
Firefox versions are 105.1
Annoying but not a train smash. Chromium works fine.

Did a longish reply but login must have timed out so post lost.
Now short answer.
firefox on cli
after killing last firefox-bin process and firefox opens
[GFX1-]: glxtest: VA-API test failed: process crashed. Please check your VA-API drivers.

after firefox opens followed by closing/exiting (don't recall if single open tab closed or whole window closed)
[2022-09-27T19:14:18Z ERROR viaduct::backend::ffi] Missing HTTP status
[2022-09-27T19:14:18Z ERROR viaduct::backend::ffi] Missing HTTP status
Sandbox: Unexpected EOF, op 0 flags 00 path /proc/cpuinfo

Some other tests involving copying an updated working, incl firefox, MX19.4 system from another PC to this my reserve PC results in the same "non-working" firefox.

I have never encountered that and a web search for "Missing HTTP status firefox" turned up stuff that, frankly, is way beyond my pay grade.

Have you tested with a different browser? If your distro does not include another GUI browser, you might check to see if it includes a text browser such as lynx or links; many distros do.

Just to rule out problems with your connection, you might also try to ping and traceroute a website to verify that you can see the Big Wide World.

AntiX includes the graphical version of links, which is a nice fast little browser and works on many sites including this one. It doesn't work on sites like gmail because it doesn't do javascript.

That error is beyond my pay grade too, but apparently firefox is talking out but not in http format. So the http status is not noted when something talks back, and firefox is noticing that, and bellyaching about it.

It points squarely at firefox. Reinstall, preferably a self contained version that doesn't rely on external libs. It's what you get when you download from their site.

Thanks everyone for the observations and suggestions which I'll try. I'll pursue some more tests myself out of interest sake. Other wise I'll be putting this issue to bed soon, solved or not.
One of my tests was copying across from another PC a working MX19.4 system which included a working firefox only to have firefox exhibit the same problems as submitted in the initial post. MX21.n.x does not exhibit this problem and functions properly.

As this problem is on my reserve PC it is not a major issue for me, merely exasperating that it has not been solved.
One test which I'll try again is to download binaries from Mozilla direct (also suggested by business kid), purge all vestiges of the existing firefox, not just un-install incl config files via Synaptic but a search of the whole system and purge. First try of this was unsuccessful but I'll try again.
Chromium works fine.