If the two browsers crash/freeze in both Ubuntu-based Pop_OS! and independent Solus OS, then the issue may be related to the way in which the browsers are set up.
In addition to checking the error logs as Pomon suggests, you might want to look at Firefox crash reports (type "about:crashes" in the address bar) and review "Troubleshoot Firefox crashes (closing or quitting unexpectedly)". The article links to numerous troubleshooting branches.
I can't speak to Vivaldi because I don't use it, but given that both Firefox and Vivaldi are crashing/freezing, I would look at non-default similarities (settings, themes, extensions and so on). Chances are that what is causing Firefox to crash/freeze is related to what is causing Vivaldi to crash. Get to the bottom of one and you will probably find the other.