This might be an OS X issue, as I have no problems on W11. When Send is added, I see the first picture, and under Options, the 'Create link for Send service instance' box stays checked when the tab is closed and reopened. Other providers show the exclamation warning, which reminds to enter account credentials (see other pictures).

When I clicked 'Find more providers' in Settings, I set the version filter in the Add-ons tab to 128, then added each provider.

This is all done with a new profile created in 128, not one used in an older version. I suggest you test with a new profile, made from Help/Troubleshooting Info, about:profiles. The current profile is unaffected.

Thank you sfhowes!

Yesterday when I was trying to add the send service the "Account Settings" (as in you first image) dialog did not appear. Have tried again today and it DID appear and I managed to get the Thunderbird send to work (although with a different "send service URL" in order to send a file larger that 2.5GB and I don't yet know if the file was received). So my problem is probably solved.

I haven't closed off my query as your mention of profiles got me to look at my set-up and I'd like to ask what is perhaps an off-topic question:. I seem to have 6 profiles: "default-release-4" which is "in use" and set to default. I am the only user of my PC, but I do use Thunderbird to access 5 email accounts. Searching with Finder (Apple equivalent of File Manger) I cannot see anything to link the profiles to individual email address. So my question is whether I can safely delete all the profiles that are not in use without losing access to any emails on the different IMAPI servers - do you happen to know? thanks Werner

Thunderbird just uses one profile at a time, so if you see five email accounts when using Thunderbird, they're all in the same profile. Other profiles can be safely removed. A profile contains the addressbook and all active email accounts.

thank you David!