I will continue to use Firefox ESR version on XP until I no longer can. There will be a point where you can no longer get updates for the ESR version or not be able to update add-ons. Security based add-ons would continue to provide a little extra security to the ESR version.
All things come to an end, but before that start considering your options, are there any browsers still supporting XP, etc.
But before all of this, start thinking about a robust backup and recovery strategy. You really should be doing this already, to recover from any computer issue, be that a crash or malware attack.
Hard disk imaging software is such a strategy to make it easy to set your system back to what it was before you experience a problem you can't live with.
I run a hard drive drive image weekly and keep copies of the last 6 drive images. The worst case scenario being I lose the last 6 days of changes. But some of the software allows for incremental updates so only new entries or modified elements which could be run daily. In this case you could be down to only losing changes within the last day.