Upgrading mid-2010 MBP to High Sierra?

High Sierra is not dramatically different from El Capitan (in terms of hardware requirements), but does offer much more recent versions of Safari. I would not hesitate to do that upgrade. Don't forget to apply ALL possible Software updates once installed, which will install an even more recent Safari.

The "approved method" before begging ANY operation that will re-write over 350,000 files is to have a Trusted Backup on hand before you begin.

I'm familiar with where you're at. I just recently upgraded from a 15 inch mid-2010 MacBook Pro (running High Sierra) to a new 2023 model. Is it worth the hassle to upgrade your machine to High Sierra? That's up to you. I, probably, wouldn't expect a marked improvement in performance. You're still hindered by the age of your hardware (in my opinion). Your issues with Safari and Firefox may stem from the fact that you cannot use the newest versions of those browsers (with the latest security updates) because of the older OS. I experienced browser issues, as well. I sought out faster browsers. (Was using Brave, for a while. Now, the old Pro can no longer run the newest version of it.) I believe Office 2016 is the newest version that can run on High Sierra (with the caveat that Microsoft is no longer supporting it). I can't comment on performance. I was using 2011 on High Sierra with no issues.

szladob wrote:

Many thanks for getting back to me! I am using time machine - would that give a possible full re-install of the old system if it all goes haywire?

That is exactly what Time Machine DOES. As long as it has completed recently and is not 'stuck' you are good to go.