Tools menu (alt+T) > add-ons.

No small text is a firefox add-on. Perhaps try https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer/

Long ago I spent a lot of time setting things up so I could read the tiny font the emails are displayed in.

v38.1.0 has SCREWED IT UP. How do I go back to the previous version?

I guess its THEME & FONT SIZE CHANGER that is the addon which I used, but its screwed up now. When I increase the font size for the text in the drop-down menus etc, the size of the pop-up pane does not expand proportionately, so I can't see everything in the pane. The pop ups need to either do that, or have a tool in the bottom right corner to adjust their size.

And trying to get the text of the emails to get bigger always seems to also involve changing the size of the Tbird informational text.

Trying to juggle the addon settings with the Tbird Pref settings... PIA & not working out.

I am withdrawing from this discussion. I have no time to listen to those who do not want to change.

Matt said

No small text is a firefox add-on. Perhaps try https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer/

If its not a TBird addon, why does it appear in the TBird ADDONs window?

No idea.

This Console Error says that THUNDERBIRD is having a problem loading NoSmallText addon:

7/24/15 6:24:31 PM [0x0-0x79c79c].org.mozilla.thunderbird[34445] 1437776671859 addons.xpi WARN Error loading bootstrap.js for nosmalltext@pjs.nl: TypeError: redeclaration of formal parameter b (resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///Users/xxxxx/Library/Thunderbird/Profiles/4g45o3sh.default/extensions/nosmalltext@pjs.nl.xpi!/bootstrap.js:444:18) JS Stack trace: @XPIProvider.jsm:4348:1 < XPI_loadBootstrapScope@XPIProvider.jsm:4348:7 < XPI_callBootstrapMethod@XPIProvider.jsm:4423:1 < XPI_startup@XPIProvider.jsm:2168:13 < callProvider@AddonManager.jsm:208:12 < _startProvider@AddonManager.jsm:669:5 < AMI_startup@AddonManager.jsm:840:9 < AMP_startup@AddonManager.jsm:2471:5 < AMC_observe@addonManager.js:55:7

also this next line:

7/24/15 6:24:31 PM [0x0-0x79c79c].org.mozilla.thunderbird[34445] 1437776671860 addons.xpi WARN Add-on nosmalltext@pjs.nl is missing bootstrap method startup

But it doesn't claim to be for Thunderbird. "For Firefox 35.0 and newer."

Small text size in email message content (which is what that add-on claims to fix) is trivial to set up.

Tools|Options|Display|Formatting→Advanced and set the minimum size to something that suits you.

Many Firefox add-ons appear in Thunderbird's add-ons search, and always have, in my experience. It's a weakness of the add-ons delivery system. Just as clicking on the "Install" button in the webpage leads to difficulties in Thunderbird too.

Older versions: https://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/

Zenos said

But it doesn't claim to be for Thunderbird. "For Firefox 35.0 and newer." Small text size in email message content (which is what that add-on claims to fix) is trivial to set up. Tools|Options|Display|Formatting→Advanced and set the minimum size to something that suits you.

Then explain this Console Error Message saying its trying to LOAD into THUNDERBIRD:

under rev 38.1.0

7/24/15 6:24:31 PM [0x0-0x79c79c].org.mozilla.thunderbird[34445] 1437776671859 addons.xpi WARN Error loading bootstrap.js for nosmalltext@pjs.nl: TypeError: redeclaration of formal parameter b (resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///Users/hal9000/Library/Thunderbird/Profiles/4g45o3sh.default/extensions/nosmalltext@pjs.nl.xpi!/bootstrap.js:444:18) JS Stack trace: @XPIProvider.jsm:4348:1 < XPI_loadBootstrapScope@XPIProvider.jsm:4348:7 < XPI_callBootstrapMethod@XPIProvider.jsm:4423:1 < XPI_startup@XPIProvider.jsm:2168:13 < callProvider@AddonManager.jsm:208:12 < _startProvider@AddonManager.jsm:669:5 < AMI_startup@AddonManager.jsm:840:9 < AMP_startup@AddonManager.jsm:2471:5 < AMC_observe@addonManager.js:55:7

7/24/15 6:24:31 PM [0x0-0x79c79c].org.mozilla.thunderbird[34445] 1437776671860 addons.xpi WARN Add-on nosmalltext@pjs.nl is missing bootstrap method startup

Zenos said

But it doesn't claim to be for Thunderbird. "For Firefox 35.0 and newer." Small text size in email message content (which is what that add-on claims to fix) is trivial to set up. Tools|Options|Display|Formatting→Advanced and set the minimum size to something that suits you.

No, I've done that and is broken in this version. I've set all the minimum font sizes to 40 pts, and un-checked allowing emails to use their own fonts.