Hi, and welcome to the Apple Community.
Thank you for telling us about your Mac model, describing the problem-situation,
and what you were doing when the difficulty first appeared. You are very helpful!
Now am wondering which MacOS is installed on your Mac,
and
which version of Firefox is installed.
We are all Apple users, like yourself, in this community (not Apple itself).
I am no expert, but no experts have replied as yet., some may chip in now we've begun.
Re: Any tips on what I can do?
Apple support has some useful articles online (links below) that may help.
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Re: "Macbook Pro stopped working.
It started while I was just using Firefox ... "
Wondering if you tried re-starting your Mac, ie: when Firefox "misbehaved".
If so, was an error message displayed.
(There can be compatibility issues ... eg: with newer Mac software and older apps that need updating.)
Firefox may also have some similar "side-effects" that Safari, exhibits at times:
If Safari on Mac doesn't open a webpage or isn’t working as expected - Apple Support
What to do:
If an app on your Mac stops responding, closes unexpectedly or won’t open – Apple Support
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Troubleshooting articles from Apple Support.:
If your Mac starts up to a blank screen – Apple Support
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Re: I have a feeling it's still booting
If your Mac doesn't start up all the way - Apple Support
How toi remove login items to resolve start-up problems on your Mac
https://support.apple.com/en-au/guide/mac-help/mh21210/mac
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Re: Its out of warranty unfortunately.
Commiserations!
Apple Support may still provide some advice when needed.
There is a "Get Support" link to Apple at top right on this discussion page.
or
Apple phone numbers around the world
Contact Apple for support and service - Apple Support
Hope there's something here that helps to, at least, begin pin-pointing the cause of the issue.
All the best :-)
