The one used by Gmail, very readable and pretty is Roboto. Since all font used by Thunderbird are the one installed in the operating system, one only need to install the above-mentioned font to used in emails.
The Font is freely downloadable from Google sites. For Manjaro/Arch Linux users this font is present in the official repositories under the name ttf-roboto.
After that you need to selected as main font for messages.
- Preferences > Composition under HTML Style select the Roboto font, with Medium size
- Select the chosen Account
- Select Account Setting, on the top right corner
- On signature text select the Use HTML option
- Append the signature
Since the font used for this is the fixed font one, I find a workaround to have Roboto also in the signature. Therefore, have consistency in the mail. Because we selected option 3, we can use HTML to personalize the text.
In my case I did the following:
<p style="font-family:roboto">
Leonardo Salicari<br>
Ph.D. Student in Complex Systems<br>
University of Padua<br>
Department of Physics and Astronomy "Galileo Galilei"<br>
</p>I used the style attribute of p to select the Roboto font. In this way the signature is consistent with the mail body.
Lastly, to append the signature only on the first email, avoiding doing so on replies, do the following:
- Select the chosen Account
- Select Account Setting, on the top right corner
- Go to Composition and Addressing
- Deselect Include signature for replies