This applies to the Threads Pane, not quoted content in a message:

http://forums.mozillazine.org/viewtopic.php?p=10177535&sid=46f64ede20fa0010fb3f60306117734c#p10177535

Help/Troubleshooting, Profile Folder, Open Folder, 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 numbers as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Options/General/Config. editor, restart TB.

#threadTree treechildren::-moz-tree-indentation{
width: .8em !important;
}

Tested and working in TB 78.8.1.

Thank you sfhowes for this recommendation. However it did not seem to resolve the issue.

The procedure was executed exactly as stated. When Thunderbird relaunched, email threads format remained unchanged with older messages becoming progressively more indented and text content increasingly narrower to point where oldest ones represented as long narrow column in which each line one word wide, literally.

Perhaps this fix effective for new thread messages only? That would of course be helpful.

But at present I'm trying to get existing thread messages to appear in readable full, standard display field format to enable easy scroll down review. Since no visible change resulting from that procedure, for now TB 78.8.1 has been reverted back to prior state.

Any further advice from anyone familiar for accomplishing the objective noted above would be appreciated.

Thank you -

The css code does work, as stated above, so if you don't see any change, it's probably because there is an error in applying it. Try a userChrome file that produces an obvious change, such as this code that changes the scrollbar color:

scrollbar {
  -moz-appearance: none !important;
    background: orange !important;
    scrollbar-width: 15px;
    scrollbar-color: grey orange;
}

Also, confirm that you are referring to the Threads Pane indentation, not quoted messages in a message.

OK, thanks sfhowes. I'll try doing it over again according to your initially recommended procedure. This issue pertains to the full message pane format as you would see displayed in conjunction with any newly received email; NOT to quoted text within a message.

Thank you again. Let's see if works on retry.

OK, apparent reason for this that text editor, Notepad, only saving as text document .txt. When "all files" (only other option) selected and saved, Notepad reverts back to .txt default every time. Apparently a way to resolve so that "all files" (and other save format options) work, but not yet quite found exact work around method for same. Thanks -

When you save the file, name it userChrome.css and select type All Files. It should appear in File Explorer as Type Cascading Style Sheet. On the View tab of File Explorer, check 'File name extensions' to ensure the file isn't saved as userChrome.css.txt.

Thank you. That all checks out. Saved as user.Chrome.css; .txt not shown when file extensions checked. And file name Type Cascading . . . . . appears exactly as indicated. Change made as advised to that config editor item. But when Thunderbird restarted no change to email thread indents and still progressive narrowing of message content as scrolling down to from most recent to oldest to point where earliest appears in long text column literally one word wide. Also, I tried changing the code element .8em to .1em with thought that doing so might reduce indentation lengths. Only other thought the perhaps not actually being saves as "all files". Suppose could try different text editor other than notepad? Uncertain what next to try in attempt in attempt to get it to work?

Did you try the code for the scrollbar? If you didn't see the change, it's because there's a mistake in your application. The chrome folder has to be a subfolder of the profile folder, not the Profiles folder or the program files folder. There's no need to use a different text editor, as Notepad is sufficient.

The scroll bar code color change works. Thread messages Indentation + text line width older messages progressive narrowing fix perhaps works only for new threads but not as pane format/display adjustment for already existing threads? Former would be great but immediate objective re. latter. Thank you for sticking with me on this.

The scroll bar color changed but the scroll bar itself actually did not work normally. Orange color at top and bottom but normal scroll function not same.

Please disregard prior message. My mistake. Checked again. Scroll bar orange working fine. Color change applied in at first slightly confusing configuration.

The indentation code works here with existing threads. The first picture shows the default indentation, the second with indentation = .3em.

Thank you. This is code used as advised - #threadTree treechildren::-moz-tree-indentation{ width: .8em !important; }

Tried .8em, .3em, .1em. Nothing seemed to change. By end of thread (earliest messages), message text one word wide. Now beginning wonder, though, whether indentation actually did decrease but message thread so long that progression to narrowest inevitable regardless. Shall try .0em; perhaps adjust to no indentation?

Change to .0em did not help. Approx, 47 messages and counting in thread. Extended exchange with escalated troubleshoot engineering team regarding issue with application suite module. Leading firm and product within particular principal market sector.