First, I cannot say I have a solution, but I see your default font for Windows is smaller. Have you tried the small er setting in Linux? I also suggest setting all fonts the same, not mixed as you show. l suggest doing this for 'Latin' and also for 'Other Writing Systems' to minimize inconsistencies.

This can be done with css:

/* header pane subject style */
#expandedsubjectBox {
  font-weight: normal !important;
  font-size: 10px !important;
  color: red !important;
}

Help/Troubleshooting Info, Profile Folder, Open Folder (Linux: Directory), close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code, change the number and color as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video on how to create a css file (Firefox and TB)

The default bold style has 700 replacing 'normal' in the font-weight. A setting of 600 or 500 is 'less bold'. Omit the color statement for black text. See picture.

First, I cannot say I have a solution, but I see your default font for Windows is smaller. Have you tried the small er setting in Linux? I also suggest setting all fonts the same, not mixed as you show. l suggest doing this for 'Latin' and also for 'Other Writing Systems' to minimize inconsistencies.

Thanks, I tried this but the default setting only seems to affect the text within the message itself.

This can be done with css:

/* header pane subject style */
#expandedsubjectBox {
  font-weight: normal !important;
  font-size: 10px !important;
  color: red !important;
}

Yes, thank you, this is the solution. I'm still in Linux at the moment, is there coding to change the text in the 'From' box as Windows handles this better too.

Code for the sender's name and address:

/* header pane sender name style */
.recipient-multi-line-name {
  font-weight: 500 !important;
  font-size: 16px !important;
  color:red !important;
}

/* header pane sender address style */
.recipient-multi-line-address {
  font-weight: 500 !important;
  font-size: 14px !important;
  color:grey !important;
}

See picture.

Thanks again but no matter what text size I set it remains 'multi line'. No problem, must be a Linux thing.

I have one more minor annoyance, this time with Firefox. Is there a way to turn off the address bar highlighter when you open a new window or tab?

I would post the highlighter question in the Firefox forum, if you don't find it in a search.