Display fonts for received and composed messages are set in Settings/General/Language & Appearance/Fonts & Colours, Advanced. Set the fonts and sizes (in pixels) with Fonts for: Latin and repeat with Fonts for: Other Writing Systems. Uncheck 'Allow messages to use other fonts'.

Just to be sure there isn't a hidden setting here you may not be aware of that could have a bearing on this, go to Settings > General, click Config Editor at the bottom, then type layout.css.dpi in the search box.

What value does it show? If not -1 (default), note the current value (in case you want to change it back), set it to the default -1, and see whether things look better. If the change makes things worse, just change the setting back to the value it had before.

Thanks, but neither of these suggestions helped. Any other ideas?

would you be able to post screenshots of your Latin settings and Other writing system settings, per sfhowes' suggestion? I have always found that to work, so I am hoping that seeing your settings may prove helpful.

Here you go.

Make them identical. they are not now. set minimum font on both and check that both have identical font sizes and uncheck fixed width fonts unless that is important for you.

Thanks David, I did that and there is no improvement.