Have you added any new extensions close to when the crashing started?
Menu > Extensions and Themes to check what you've got added.
Try first tapping the ⚙ then "Check for updates" to see if any are outdated.
Failing that, try turning one off, close Firefox, wait a minute, bring Firefox back up and try sites you know have an issue. If you still get Firefox randomly disappear, re-enable that add-on and disable another.
Could be a graphics-driver if you're mostly on online game or video websites before it disappears:
Menu > Settings > scroll-down to "Performance"
Un-tick "Use recommended performance settings"
Un-tick "Use hardware acceleration when available"
Fully close all Firefox windows, wait a minute, then bring it back up
Videos will now be slower and choppy, especially at higher qualities (1080p+) and if sped-up
Many high-end online games will fail to run, or be very-slow
But if Firefox doesn't disappear, it's likely a graphics-card driver causing the issue; do a Google and look into how to update it (or just let Windows Update pull you one down, if that's what you're PC uses)
Your device could be running out of RAM
Apps disappearing without-notice is quite-common if all the RAM in your device has been filled, and you have the "pagefile" or "swapfile" for your OS disabled. In modern Windows, the "Performance" section in Task Manager shows your current RAM use; if you click on an internal drive there (e.g. "Disk 0" or "Disk 1"), look for "Page file". If it says "No" on every Disk you click on, you have no pagefile turned-on, so once you fill-up all your RAM, apps just disappear as they can't show an error as there's no memory left to!
