I'm having the same problem, and I haven't found a solution for this yet. As a workaround I'm temporarily using Outlook to remove the attachment.

a digital signature guarantees the mail as presented has not been modified since it was signed. This is done by way of a checksum on the entire email message body (attachments included)

SO by definition changing the mail by removing content, be it an attachment or substituting new text in the email is not possible with signed or encrypted mail.

I am sure you used to be able to remove the attachments and break the signature. But I just tried and it no longer does that.

Thanks for your answers, christ1 and Matt.

Yes, using Outlook is a good temporary workaround. I'm wondering if there is a "solution" also for Thunderbird.

For me, it's ok to break the signature for _archived_ emails in Thunderbird. I tried with the Thunderbird add-on "AttachmentExtractor" but that doesn't seem to work either. Would anybody know of another way to do so?

I considered it yesterday, and I think you should file a bug and call it a regression. When I was looking for one yesterday I did find a couple of bug to support my aging memory.

Here https://bugzilla.mozilla.org/show_bug.cgi?id=288700

The discussion is about what to do about the signature and that is is broken post detach.

And here https://bugzilla.mozilla.org/show_bug.cgi?id=2920 (1999) were the detach feature was implemented in Sea Monkey or at the time it would have been called Mozilla Suite.

If you file a bug, please post A link. I would like to follow that one.

I added my comment to the previous bug:

• https://bugzilla.mozilla.org/show_bug.cgi?id=288700

and also filed a new bug to increase priority:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1282701