Fennec being a delayed build has been a thing for years at this point: it's a pain in the ass to get built and in f-droid. I mean, just google 'fennec f-droid out of date' and you'll see people talking about this going back to 2020.

I didn't exactly find a stunning shocking unknown thing: Fennec is slow on builds, it got outdated, there was a zero-day in older Firefox versions, and so bam: there's a security issue in Fennec.

Might be worth adding the Firefox security RSS feed for anyone using Firefox or a derivative browser so that you've got the best information about issues like this.