Writing email, or replying/ forwarding them?

What often happens is programs like Microsoft Outlook and ignorant but well meaning users specify font sizes in points or pixels. The result is when you reply "inline" that their specification sort of get run into your reply making font changes hard.

So before making font changes make sure the selected text is "body text" or "paragraph" before you even think of other formatting. That will force the font issue and mostly fix it as it removes the preformat and other users text styling.

Here's an example of the frustration I'm having. The first screen shot, I highlighted an area of text. ''''You can see the setting: paragraph, Arial, san-serif, and medium font size''''''. The second screen shot I changed the font size from medium to large, nothing else. You can see it only affected half of the highlighted area.

This is not exactly relevant to your issue here, but I want to point out that the Theme & Font Size Changer add-on is intend to adjust fonts and sizes in the menus and lists in Thunderbird. It is not intended to have any effect on message content.

Something I noticed, I am running T'Bird 52.9.1. but I see a version 60.0 available, however if I click on "check for updates", it tells me I'm up to date. I also noticed I have a 32 bit version of T'bird, but I have a 64 bit OS. Could this make a difference? If I upgrade, will I retain all my emails?

you will get updated in due course, currently TB 60 is not being offered as an upgrade until the drivers are satisfied there are no glowing errors or omissions in the download. Not starting the upgrade process is a way to minimize the risk if the new version has a serious undiscovered bug. Despite extensive testing. But it is not all that relevant to this discussion.

I see you are highlighting your signature. That is itself a possible cause of your trouble. How was it created, does it contain font definitions (just a a explained about replied and forwards). Are the fonts defined directly or using styles. lots of variables and no answers because we has a random bit of "code" included in every new composition.

The signature is an HTML code. As described here: https://www.groovypost.com/howto/howto/create-html-signatures-in-thunderbird-without-learning-html/

It would seem you are using something to write the word 'Stu' above the highlighted section.

Highlight the entire signature - start from above the 'Stu' part. Insert > HTML A window appears. post image of that window showing HTML

Also double check and confirm that HTML is identical to what is shown in the Account Settings 'Signature text' box - you could post image showing that as well.

The "Stu" is an image. Here's the screen shot of the HTML and the Account Settings Signature box.

The account settings does not match. The line starting with "scr=" goes on forever! It's probably at least 50 times longer than what you see there. (note the slide bar under the box)

I've tested on a system using same info and not using the addon you mention and all is ok.

You need to understand that the Theme Font & Size Changer allows you to change the global font size and font family for the user interface in Thunderbird. It should have nothing to do with what you are talking about, which is how a 'Reply' in new Write message window has signature text strangely operating when needing to select different eg: font Size.

However, it would be valuable information to see if there is anything weird going on.

Please perform a test: Disable the 'Theme Font & Size Changer' addon extension, restart Thunderbird and repeat exactly the same as before. select to Reply to the same email you received from 'Matt's comment'. highlight same section and attempt to enlarge font. Does anything change? Do you get exactly the same issue?

I think you nailed it! With 'Theme Font & Size Changer' disabled, everything seems normal again. I can adjust the font size up and down with no strange reactions. I feel silly that I didn't try that. Thanks for your help. Is 'Theme Font & Size Changer' a Mozilla creation or a third party add-on?

https://addons.thunderbird.net/en-us/thunderbird/addon/theme-font-size-changer-for-tb/

Addon extensions are created by any person who wants to put their time into creating them. Baris Derin created 'Theme Font & Size Changer for ThunderBird' 64.1

They are not created by 'Mozilla Thunderbird'. Authors are responsible for maintaining their addons and often rely on donations to afford time to do this.

It is always worth while checking what addon you are using and whether it claims to be compatible for the Thunderbird version you are using. Some addons will continue to work on TB versions newer than the one stated in the addon webpage download, others do not.

You should check what addon version you are using and whether it is up to date to use on the specific version of thunderbird.

Should you encounter an issue, it is advisable to offer full info to the person who authored the addon extension. What OS you use, version of thunderbird, state name of addon and it's verion and explain clearly what anomally you experience. Also inform them that disabling the addon clears the issue. You can locate their support email address at the addon link at the top of this communication.

Thanks for your help.