See if the info at https://support.mozilla.org/en-US/questions/1126211 helps.

Thank you for your reply, but no help. That link is 8 years old and I have already been through that post and/or several similar years-old posts. Thunderbird must have changed much since then because I have not found in v128 some of the features that those old ones mention : - no Preferences in 3-bar menu, hence nowhere to "Enable Auto-reply" - "Reply with Template" not listed in Actions in Manage Message Filters

Looks like Thunderbird has removed auto-reply, which is a shame because I'd find it's fine-grained selection of particular incoming addresses far more useful than gmail's coarse catch-all auto reply.

And nowhere to speak with Mozilla themselves to check whether they have removed auto-reply.

I set it up following the instructions and it worked fine.

I set it up following the instructions and it worked fine.

Sorry, I've been away all week.

You're right. I tried again from the start and this time a template was saved in the Templates folder - I don't know why it wasn't saved before - following which "Reply with Template" appeared in the Actions under Manage Message Filters and the saved template became visible and selectable.

Thank you for confirming that the process did actually work OK.

Cool - you're welcome :-)