There's been a lot of discussion as to why TB does this, which apparently has to do with the underlying HTML tags. Several years ago I found and installed the 'Change Quote & Reply Format' extension, which makes the fonts much more stable. However, it seems that TB can no longer find it when you search for add-ins. Instead, download it directly from the developer's web page https://freeshell.de/~kaosmos/changequote-en.html

I will warn you that the extension works, but recently added its own idiosyncrasy: when TB conducts an auto-save of your message, it moves the cursor to the beginning of the message. I've not been able to find a fix for this and the extension developer has not updated the web page in a long time. So I just set auto-save to 15 minutes to reduce the frequency of the annoyance.

Hope this helps.

Thanks markstetz for taking the time to answer. I'll certainly check it out. I've taken to just typing the whole letter, letting the font size do whatever it was going to do, then highlighting the whole thing and changing the font on the whole letter all at once before I send it.

It looks perfect and fine when you do that, but several times when I've cc'd or bcc'd myself a copy it not only changed size of some of the font that originally changed size as I was typing, it's even changed the type of font itself. So I end up with a couple of paragraphs that look the way I wanted and a couple that stand out as a totally different type face right in the middle of the letter. Assuming the folks on the other end are getting the same thing I'm seeing in my cc, they must think I'm either stupid or nuts. Again, very frustrating, and this is not an unimportant problem, especially if it's a business mail. It makes the sender look VERY sloppy!

Thanks again for your time. It's greatly appreciated!

Ty

One alternative when it's worth the effort: copy and paste your typing into a word processor to make the font and size uniform before copying and pasting back into the note. Pasting into a processor also reveals subtleties not readily apparent.

Good idea WBjose, and thank you. I'm going to try doing that in Open Office when the email is an important one. We've been unfortunate enough to have to write lawyers recently and when they respond to something I sent them and my original has 3 different font types in a short letter I just cringe, because this bug makes your original note look perfect when you hit the send button. You have no idea you are sending out something that makes you look both sloppy and stupid. Your idea would work well for when I send those important mails.

It really amazes me that TBird doesn't seem to care about this as, to my mind, it's a real problem and a really big bug.

Thanks again for taking time to answer and share. It's really appreciated.

Ty

Please try the current beta. There has been at least on font bug back ported in the very recent past.

Bug 1159834 was in the 29th April change set so should be in the current beta4 as it was build on the 2nd May

Thank you Matt. I've never downloaded a beta version here before, but this bug is driving me crazy. I'll give it a try. Fingers crossed and thanks again for the help.

Ty

It is only about a week or three from release so it is VERY stable, and feature complete.

I have the same problem but it's been around for a long time. I agree with tygrrr that it's time Mozilla did something to fix it. I'm considering switching to Outlook Express.

I've been using the 38.0b4 beta for about two weeks (Mac/OSX Yosemite). Marginal improvement in that the font still changes depending on where I insert the cursor - even in a new message. But perhaps less often than before. Re-creating my HTML signature template(s) helped a little. Am downloading 38.0b6 now and will see if that behaves better.

TB is on version 38 and one still can't compose an e-mail without the editor being flaky? I used to use Pegasus on a W95 platform that didn't do that.

All I'm shown is that I'm using beta v. 38.0. Font size during composition of a note seems to be steady. However, my specification to use a certain size font for composing notes is not honored when composing a note while forwarding or replying -- a smaller font is the default value, forcing a manual highlighting and size increase. So, making progress, just not there yet.

You do realize that the font you specify is in pixels don't you. so is 30% smaller that a word processing font in points.

Matt said

You do realize that the font you specify is in pixels don't you. so is 30% smaller that a word processing font in points.

Matt, everyone has been reporting this issue for a long time. It has zero relationship to what font size you set up or whether you know that you're dealing with pixels.

Example: A few minutes ago (in Tbird 31.7.0) I typed a 1-line response to a friend's e-mail. I did not go back and insert any text, did not copy/paste, did not save a draft, did not even pause to scratch my chin while typing. The e-mail looked clean. After sending it, the result showed a drop in font size in the middle of a word, which persisted through the rest of the response.

This problem did not exist in earlier, more "primitive" versions of Thunderbird. I have tried all of the recommended solutions -- including installing the add-on Extra Format Buttons (which is useful and permits setting the font size in pixels) -- all to no avail.

On a Mac: I have found that if I set my font to "Small"(which is pretty small) I can usually type and keep the same size font. When I change to a default larger size font, at any time in the email it goes to a smaller font by itself. So I have been using the smaller font and have not had the problem- Sure wish they would fix this- The smaller font is harder for me to read.

steinr98 said

On a Mac: I have found that if I set my font to "Small"(which is pretty small) I can usually type and keep the same size font. When I change to a default larger size font, at any time in the email it goes to a smaller font by itself. So I have been using the smaller font and have not had the problem- Sure wish they would fix this- The smaller font is harder for me to read.

At least one bug around this issue if fixed in Thunderbird38. Have you tried it yet?

No, I have not- I will now!! thanks