Is this a plain-text (".txt" file name) or HTML (".html" or ".htm") signature file? Also, you are saying that not just new messages show the signature problem, but also any previous messages (in the Sent folder?) which were fine with 24? That should be important to distinguish composition vs. display issue.

If you uncheck the "attach signature from file" box and copy-paste the contents of the file into the text field directly, does it make any difference? Make sure to check or uncheck the "Use HTML" box appropriately.

What you describe looks like a plain-text signature which is treated like HTML, thus only explicit <br> tags would cause a line break, otherwise the lines are just concatenated.

It is a file with an .html suffix and HTML formatting.

Yes, previously sent messages that showed the proper formatting now show the wrong formatting in TB31.

I just sent a test message to both a friend's gmail account and my account. The gmail account displays it properly, my account at the Yahoo portal webmail receives the message and displays it without the proper formatting also.

If I copy the file into the box and check "Use HTML" it is displayed correctly in TB.

Well, using the text box might be your workaround for the time being. Apparently there is some tag in the file throwing Thunderbird off course, which was just now exposed by some change in the HTML parsing and rendering from 24 to 31 (lots of "some" here, but without seeing the file contents itself, hard to tell).

You could have a look at an old message exhibiting the problem, compare it to one of the new ones which resulted from using the text box instead. Use Ctrl+U to see the message source in both cases, find the signatures, then try to figure out which tag is present (or different) in one vs. the other and make respective adjustments in your signature file.

rsx11m, thanks for all your advice. I need to find out how to write HTML, I guess, there are other niggling issues with the signature formatting like fonts and sizing. Am I recalling correctly that there was some "device" here that allowed folks who are not coders to do that?

I am having an unusual number of problems with TB31, worse than all the other upgrades, to my recollection, and I think when I first started using TB it was TB3 or TB4. At this point the ones I've noticed have to do with composing messages.

I can't be the only one. Is there some central place or clearing house for TB31 problems? Maybe find fixes or workarounds other folks have come up with?

Thanks again

You are welcome. This article has step-by-step instructions on how you can use Thunderbird's own e-mail composition window to create the HTML code needed for the text box, thus you can just copy-paste it without actually knowing what it means (for troubleshooting, obviously, some knowledge of HTML is helpful to figure out where the problem is).

I'm not aware of a central list of most common problems (yours is the first post I see on this specific issue), but you can browse topics here or on mozillaZine and ask for support at either place. Despite the almost empty release notes, quite a bit has changed (visible or invisible to the user).