When you click on Forward it opens in a new Write window. Is all the content displayed in that window ? Does content look like plain text ?

Or does it look ok at that point, but when you send it the recipient does not see remote content ? Does the recipient only see plain text? Does the copy of sent email in the 'Sent' folder look like normal with remote content or as if remote content is disabled or does it look like plain text etc?

It would help to offer info on the above queries as it may help explain at which point the remote content is removed.

If you click on Forward and it looks ok in the Write window, but the copy in Sent looks like plain text then you have created email using HTML but not sent it using HTML. This also means the recipient will only see plain text. Check settings : Menu icon > Settings > Composition Under 'Sending Format' - select 'Both HTML and Plain Text'

If you had originally selected to view Remote Content in that email by clicking on 'Options' and selecting 'Show remote content in this message' then the sent copy in Sent folder will not display remote content because only the original email had remote content enabled.

It looks like the problem is solved by checking the "allow remote content in messages" box. It looks like I need to keep checking that box after every update, as when I updated just now to Thunderbird Version 102.2.1 the "allow remote content in messages" box was unchecked again. Maybe this is a default setting that someone wishes to override when updates occur?

The "allow remote content in messages" option is not something I would advise.

Not all images are large enough for the eye to see, so you maybe enabling a pixel sized dot which then sends info back to sender. Disabling protects your privacy because spammers can verify your email address by detecting if you viewed a remote image in a message from them. Its also possible to embed an executable (malware) in images.

Advise you make sure your mouse cursor never hovers over any content of any email and accept you may get targetted as the sender will know you opened mail.

Please be aware that even if you have a safe sender - you recognise the email address of person who sent email - you cannot be certain that they actually sent the email until you open and read content. Then you would think - that looks like the sender sent it or that's a bit unusual.

https://support.mozilla.org/en-US/kb/remote-content-in-messages