Mozilla sent Firefox Version 149.0.1 to the Release Channel.   Fixed Fixed text contrast issues in the sidebar with some dark themes. (Bug 1971487) Fixed a startup crash experienced by some users caused by DLL injection. (Bug 1973947) Update: To get the update now, select "Help" from the Firefox menu, then pick "About Firefox".  Mac users need to select "About Firefox" from the Firefox menu.  For non-English versions, Fully Localized Versions are available for download.   Release Notes

Yes I already tried a reinstall of LXQt and I'm sure it's OK on its own because I can launch it without the DM. The latest update to Arch split the linux-firmware package into several sub-packages. I don't know if that introduced a regression or not. Or maybe one of the video packages could be faulty. I haven't tried SLIM but honestly I am inclined to wait for a few updates before I try anything else. Right now this is on the oldest and most useless laptop I have, and it still works fine if I just log in in a TTY and then start the GUI with startx in the .xinitrc I'll also follow the discussion in the Arch Linux forum. I can't be the only user of LXQt in Arch with LightDM. Thanks for the suggestions.

conclusion....this is an Arch issue!   More specifically, an Xorg or LightDM or LXQT issue within Arch, but localized to Arch.  I'm thinking that pacman either sync'd a corrupted package (not likely), or deleted a package during sync (more likely), or the package got corrupted (most likely).  Have you tried re-installing LXQT?  Maybe even forced installation of missing packages with -f flag (I'm using Debian convention....not sure what the Arch equivalent would be?).   We've also established that LightDM and SDDM (the recommended DM for LXQT) both provide the same error.  What about an older "generic" DM like SLIM?

Computer: Toshiba Netbook Distro: Arch Linux DM: LightDM Server: Xorg Desktop: LXQt Result: Fail   Computer: Lenovo Thinkpad T430 Distro: Debian Trixie DM: LightDM Server: Xorg Desktop: LXQt Result: Pass

I just logged in in Debian  Trixie LXQt from LightDM and it works fine. Go figure.