I upgraded from 39 to 40 and I think the only issues I had were:

• Background I was using got removed, got a better one anyway
• A few gnome extensions stopped working and I had to update them or find an alternative
• Had to re-create virtual disk mapped to real disk for booting windows installation in virtual box (there is a sonicwall VPN I have to use for work which only works on Windows)

I think that was it!

I have had some strange behaviour from Firefox saying it’s become unresponsive a few times and at the same time Thunderbird but that seems to have fixed itself now

That’s an Irish word meaning Chief but the English spelling of it. A lot of place names in the UK are badly anglicised versions of older names from ancient languages/dialects which is why they often pronounced differently to how they are written in English. And often it’s only people in the local area who know how to say it correctly unless it’s particularly famous or well known for some reason!