wtf is arisu becoming, why has no one given a serious answer?
what OS are you running? I'm guessing it's windows since you have this issue. I recommend you format your computer. If you don't want to do that install https://www.malwarebytes.com
since it most likely not your browser that is the issue but you have malware thats messing with firefox. It might be some extension being installed even when you reinstall firefox so maybe try another browser just to see if it keeps happening.
You should really format the computer though I wouldn't trust it with anything sensitive without formatting it and even then I'd still not trust it with anything too sensitive like cryptocurrency wallets or signing in to any email that doesn't have 2FA enabled.
best of luck to you arisu