Presumably you're using an add-on for this. Contact the add-on author or look for an alternative. https://addons.thunderbird.net/thunderbird/

Not using add-ons. Tried unsuccessfully to d/load Image Resize add on. I'm using Win 10 and used to be able to click on 'send to mail recipient' but for some reason this now asks for a profile. Is there a Firefox add on enabling me to reduce photo size as part of the email process (as I could do before)?

Not using add-ons.

Thunderbird has no built-in functionality to resize an attached image.

Tried unsuccessfully to d/load Image Resize add on.

You'd need to be more specific about what 'unsuccessfully' means.

I'm using Win 10 and used to be able to click on 'send to mail recipient' but for some reason this now asks for a profile.

I'm not guessing where you click and using which application.

Is there a Firefox add on

Firefox uses Webextensions which don't work in Thunderbird (yet).

Is there a ... add on enabling me to reduce photo size as part of the email process (as I could do before)?

Thunderbird add-ons are at https://addons.thunderbird.net/thunderbird/

Note, not all add-ons may actually work, despite claiming compatibility with TB60.