This usually means that a message encoded in, say, utf/unicode, is being displayed with a different encoding.

In the old menu, View|Text Encoding. If it's set to "western" try "unicode", and vice versa.

Thunderbird ought to be be able to read the encoding from the message's headers and switch accordingly. I've never understood what leads it to decide incorrectly.

I'm afraid it acts the same with either coding.

or it is someone using Microsoft smart quotes in email, not realizing that the character sets for email simply do not have those things.

Often they manage to get it in there by pasting from word. Other your Microsoft Outlook which is quite happy to format the message in a manner only legible to other outlook users.

Good point. Pasted from Word:

“smart quotes” ‘single smart quotes’ Word’s apostrophe

but when pasted into an email message and sent to myself, it all works fine, until I force it to "Western" (I use "Unicode" by default) when it appears like this:

“smart quotes” ‘single smart quotes’ Word’s apostrophe

Interestingly, the same message was also copied to a backup gmail account, where it has been re-encoded (into a "Windows-1252" character set) and now appears correctly in "Western" but with the black diamond glyphs standing in for the quotation marks when "Unicode" is forced.

�smart quotes� �single smart quotes� Word�s apostrophe

So the email provider is also possibly part of this conundrum.

But in all these cases, Thunderbird automatically chose the appropriate text encoding system to display it correctly.

The problem has nothing to do with MS Word as it appears universally in all emails, even those from advertisers.

Tools|Options|Display|Formatting->Advanced

Is the "Allow messages to choose their own fonts" checkbox ticked or unitcked? Try with it ticked. I'm wondering if somehow you have an old font which doesn't support unicode.

And this same question applies to Firefox if you have similar issues there, though the route through the menus is somewhat different.

This box is checked.