The system details for your post say:

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.181 Safari/537.36

…so it appears you're using Windows. The add-on is not supported for Linux, IIRC, where that message might be more relevant. Can you confirm you saw this message on Windows?

There are two other ways to go about this:

• set up a userChrome.css file
• use the Config Editor and adjust either layout.css.devPixelsPerPx or layout.css.dpi

BTW, I've noticed a lot of posters here seem to be using Safari. If so, what's its appeal?

> "This add-on is not available on your platform."

I don't think this refers to an OS, but rather to the browser being used. (And I don't believe the addon is limited by OS.)

You could try https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer-for-tb/ - a modified version that does not expire from http://www.rossde.com/ThemeFontSizeChangerNoexpire/index.html

right+click the "Get It Here" link, and use "Save link as" to save the XPI file to disk. Then in Thunderbird, at Tools > Add-ons, click the gear and pick "Install add-on from file"

Thank you very much Wayne, that helped, I could add the TF changer.

But, how do I then add the "theme and font size changer" to my "write/message pane", i.e. when I start a new message ("write" pane): the font is very small and I want to make the writing/reading bigger (without permanently changing my writing font in the 'option' section in 'settings')

Thus, what I would like for example, is that: i. the font size that my receivers read, is permanently 12, and ii. my reading/display font size, in which I write and read, is permanently 14 or 16 (because of my eye sight).

Is that possible ?

ns. I do know about 'smaller and larger' option within the 'writing pane', but I would like to permanently set my write pane font size to i. and ii. ... if it is possible.

Thanks !

This can be set up the Display options to say use the font and font size you like. This is not hardcoded into any emails, so your recipients can choose whatever they want.

this sets the auto display and auto 'write' font, but only for your viewing.

• Menu icon > Options > Options > Display > 'formatting' tab
• Choose the default eg: Arial
• Choose size: eg: 14 or 16
• Click on 'Advanced' button
• set all the font sizes including the minimum to 14 or16
• click on OK

To ensure a medium font size is used in sent mail:

• Menu icon > Options > Options > Composition > 'General' tab

HTML

• Font : choose 'Variable Width'
• Size: choose 'Medium

The above is usually set by default

This will not hardcode your settings in emails allowing recipients to view as they desire.

• click on OK

This will not show effect in any already opened Write window. so close any that are already open. Open a new 'Write' messages and type some content to see the effect.

see images as guide

Thanks Toad, that helped.