User Agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20100101 Firefox/17.0 Build ID: 20121128204232 Steps to reproduce: I wrote normal English emails with words separated by spaces and sent them. The emails were in HTML format. I was running Thunderbird 17 on Windows XP Professional with SP3 and all maintenance applied. The problem does not occur when I am formatting normal ASCII mail. Actual results: Some of the spaces between the words disappeared on the copy of the email saved in my Sent folder and received by addresses. I'm a programmer myself and with some guidance from folks on the mozilla.support.thunderbird newsgroup on Usenet (see "Some of my spaces are disappearing" thread, started 2012-12-16 by Rhino), determined that superfluous font tags are being generated and that spaces disappear in the Sent mail when the <font size="-1"> tag appears immediately following the last letter in a word and is followed by a single space and then the first letter of another word. In all other cases, the spaces typed in the original email are preserved. The attached file shows the HTML that was generated by my email in the Compose window as it appears just before I sent it. Expected results: The spaces should not have disappeared. I think that if the superfluous font tags were not being generated, the problem would not occur in the first place. I never explicitly set or changed the font or font size during composition of the email so the font tags should not have been generated.

Suspect this is a Core Editor bug where <font> tags are being salted randomly into HTML mode composition. Believe this needs moving to Core to get on their radar. Message-ID: <B_OdnZD_deBj9FPNnZ2dnUVZ_sydnZ2d@mozilla.org> is the thread referenced by OP.

I tried working around the HTML composer problems by using plain text only, and I found that spaces are being removed here and there. I have no idea if the mechanism at work there is related to this bug (if, for instance, plain text messages are generated from a badly tagged HTML source).

https://hg.mozilla.org/mozilla-central/rev/8a5523cc2812 was for bug 823253.

See also bug 821215

Is this specific to forwarding in the mail client? Can anyone reproduce in, e.g., <http://www-archive.mozilla.org/editor/midasdemo/>? Also, can anyone reproduce in Thunderbird 24, which has bug 812638 fixed? This sounds like it might be related, because of the random useless font tags. Probably there's some other underlying bug here, but if the bug is no longer exposed in the most recent versions, this doesn't have to be prioritized.