I’ve actually been having the same issue for a while now with Iceweasel and now Firefox. I’ve opened a ticket on the HaikuPorts GitHub about it as your post reminded me, just I hardly upload files so I forgot about doing it. It seems to not have anything to do with the Nebula driver as it happens to me with or without it installed.
The ticket I opened is here: Iceweasel and Firefox 153.0.3 crash with 'Illegal Instruction' error when trying to open a file open dialog to upload files. · Issue #14580 · haikuports/haikuports · GitHub
EDIT: Turns out there already was a ticket open from April 2025 that I didn’t see in my initial search so others have been having it for quite a while: Haiku latest nightly iceweasel exits when clicking the button to upload a syslog · Issue #12227 · haikuports/haikuports · GitHub
The way to fix it is in this forum thread: Firefox (all forks) randomly crashes on my laptop - #18 by 3dEyes
The following terminal commands need to be run to delete the files to fix the issue:
rm -rf /boot/system/non-packaged/data/mime
rm -rf /boot/home/config/non-packaged/data/mime
That did the trick for me and now the file open dialog works.