https://support.mozilla.org/en-US/questions/1269224#answer-1253658

Config. editor in Linux: Edit/Preferences/General/Indexing or search Preferences for editor.

I'm sorry but this doesn't solve my problem. I've tried this but while it certainly increased the print size on each individual email BUT not the stuff on the Main TB page, listing emails, senders, dates etc!!! The print on the latter still remains very small. And worse - I upped the print size to 2.5 in the belief and hope that this would solve my Main Tb page but it only made each email enormous......so enormous that I can't undo the 2.5 input because I can't get the "risk" box back! Please, please, please help me. I've spent days trying to solve this problem.

Can you please suggest an on-line source that I could confer with to customise my Thunderbird please? All my searches are very technical (to me). I'm more than happy to pay for this tuition and feel it's terrible that so many knowledgeable Mozilla/Thunderbird supporters and users are bursting to help while 76-year old, tech-friendly newbies like me are only dying to pay them for their knowledge.

Changing the preference does increase the fonts in the Threads and Folder Panes on my system, but if you prefer, there is a (more advanced) method in the cited topic that applies userChrome.css. If you apply a css file, you must toggle toolkit.legacyUserProfileCustomizations.stylesheets to true in Config. editor.

If you've increased the preference to the point that makes the editor unusable, see this topic:

https://support.mozilla.org/en-US/questions/1349047

There are many topics on this and similar forums that give examples of customization. Most of them require only simple copy and paste of css code. One way to find examples is to google this query:

thunderbird userChrome.css <enter the element, such as folder pane font, threads pane background color, quote colors etc.>