"unresponsive script" messages

[HemiDemiSemiquaver]

The ads are already loading slowly with/without ad blocker - certain ones just seem to takes ages. Then some of them do actually load, but others don't and a message saying "unresponsive script" comes up, with the usual options of stop script, debug script, etc. Happening quite a lot more often lately.

This is Firefox on a Mac.

[LornaMumsnet]

Hello!

So sorry about this - it sounds really annoying.

Would you mind uploading a screenshot for us?

[HemiDemiSemiquaver]

Here is one example:
(not all the episodes end in the message box; sometimes it just spins the cursor for 30sec and gives up on its own without needing to click).