In Tools/Options/Composition/General, it's not recommended to change the defaults in the HTML section. But to change the fonts in the messages you read, the relevant settings are under Tools/Options/Display/Formatting:

https://support.mozilla.org/en-US/questions/1239826#answer-1171483

Thank you for your reply. I have implemented the recommendations you made. I also re-installed Version 60.3.

However, the issue remains, that is: if I choose a particular font size in the "compose window" then this font size appears in the "sent window" as well as in the "received window" slightly larger.

If I copy the text back from say the "sent window" in a new "compose window" the font size returns to the same size as in the original "compose window".

Look, it's not a big issue and I can live with it. I just thought if other users have experienced the same problem I be interested how they have solved it. Thanks again for your advice.

I don't see that particular difference between the composed font and the received font, but it may be due to the difference in how the font sizes are defined. In the Tools/Options/Display/Formatting/ Fonts & Colors/Advanced settings, the Size is in pixels, while the Format/Size in the compose window is Tiny, Small, Medium...Huge. In Advanced, I have 16 for the Size of Proportional and 14 for Monospace, and Min. font size is 16.

This could be the explanation. In Advanced settings we have a clear definition of the font size in pixels. Whereas in the compose window the definition is not well defined by Small, Medium etc.

In Advanced, I now have 12 for Proportional, 10 for Monospaced and 12 for Minimum font size. I'm satisfied. Again, thank you for your kind assistance.

I'm completely floored. No matter what I do with the font settings the font size I see when I compose a message is much smaller than it appears to the recipient, or in my Sent messages. I'm running tests by sending messages to myself, but whatever I do this is the case. I can see that it is the same to recipients of my messages - when they reply and include my message my text is much larger than theirs. This is driving me nuts.

Did you follow the suggestions above? If you do that and your recipients have problems, keep in mind that the settings on their programs can completely override the fonts and sizes you send them. It's OK to adjust the Display options for your requirements, but it's not recommended to change the default Composition options, or to apply non-standard fonts or styles when you're composing a message.

Thank you for your advice. However, the font in my "compose window" is still somewhat smaller than in the "sent window". Anyway, I can live with that. The font I use is Verdana, optically very pleasing on the eye. I like it.

The difference may be due to sizes being defined as pixels in Display options and 'medium, large etc.' in Composition, as noted above.

OK after sending a dozen test emails with various settings I've now got a combination which works. For the record, I've got my proportional font (Arial) size set to 16 in the Display settings, and my Composition size is Small. With those settings what I see as I compose the message, and what appears in Sent items (and what the recipient sees) all pretty much match in size - which is the basic requirement IMHO.

I use Outlook for work, and have recently changed from Outlook Express to Thunderbird for personal email, and to me the font size handling seems obscure and unhelpful by comparison. In Outlook and Outlook Express if I compose a message with a variety of fonts & point sizes then the way it displays as I compose it is identical to how it appears in Sent items, and also identical to how it appears to the recipient (at least when the recipient's mail client is one of Outlook/Outlook Express/Thunderbird/iOS Mail/GMail webmail/PlusNet webmail). No experimentation of configuration settings is required to achieve this.

While I'm grumbling, why when sending an HTML message are the font size options not in point sizes as they are with most other mail clients (rather than Tiny/Small/Medium etc.)?

To sum up, the way Composition and Display font sizes interact means you have no way of knowing (without some experimentation) what the message will look like to the recipient (yes I know it depends to some extent on their mail client, but mostly they do work sensibly). Whatever happened to WYSIWYG.

I agree, the font settings leave a lot of room for improvement, and I am still learning about new features in this area. If the settings were hardcoded, many would complain about lack of flexibility, and would be unwilling to experiment and plow through the documentation, such as it is.

http://kb.mozillazine.org/Font_settings_in_Thunderbird

If the default settings were such that what you send looks the same as what you compose then 99.99% of people would be happy. Those that aren't could then tinker with the settings - or just be able to peg a zoom in/out level for the compose window.

I'm so frustrated about the font and size settings. While I type my text and copy/paste some text from elsewhere, the font and size are different, so you would think that like in outlook, you can select all the text and resize it to for example verdana and normal. Not so, the different bits of text remain different. So resizing and changing per text bit works and it then looks al the same to me.

However that is just so misleading, the recipient that gets my mail sees the text in different size/fonts ??? When the recipient replies I now get to see the different fonts and size as well.

I do like thunderbird because you see the mails of all acounts in one window, and you can set a main password for thunderbird.

Hi Hans, sfhowes's post was marked as the solution by the person who asked the original question (colortec). Your issue may have similar symptoms, but it is likely a different cause/solution. You should use https://support.mozilla.org/en-US/questions/new where volunteers can get more details about your setup.

Because this thread is solved and more people might start using it for their own questions, I'm going to lock it.