#dateValueBox {
      display: none
    }

    /* 
    #header-view-toolbox,
    #otherActionsBox {
    display: none !important;
    }
    */

    #expandedHeaderRows {
      padding-top: 0px !important;
    }

'''[SOLUTION provided below]''' [Update: It seems a lot of people are having trouble with this. I'm happy to have found the solution just now... read on.] I accidentally installed Thunderbird 115 last night, thinking it was a Firefox update. This time I can't revert to the prior version, because I don't have a recent backup of my profile folder. So I'm stuck with Thunderbird 115. The only major problem I see so far is, when I open a message in a new window, '''the Reply and Forward buttons are missing''' from the toolbar. Further, when I click on Customize, there is no way to add those buttons to the toolbar. My version of Thunderbird: 115.4.1 on Mac '''NO ADD-ONS or THEMES installed... just default.''' After digging around in Google, I decided to try Troubleshoot Mode (aka Safe Mode). To my surprise, the buttons appear in the header, however not in the toolbar. See attached screenshot. Note that this is new functionality in version 115, as noted in this article: https://blog.thunderbird.net/2023/08/make-thunderbird-yours-how-to-get-the-thunderbird-115-supernova-look In the start of Troubleshoot Mode, I tried "Reset toolbars and controls" and "Make changes and restart" – this did not solve the problem. Now the question is, what is different about the settings/configuration in Troubleshoot Mode vs. normal mode? I found no settings in Settings > Config Editor that are different in either mode, except for ''extensions.pendingOperations'' and ''mail.uifontsize'' (I exported all 6,502 of them and did a diff). ''extensions.pendingOperations'' is false in normal mode and true in Troubleshoot Mode. Unfortunately, when I change it to true in normal mode, it doesn't stick – when are restart, it's back to false. Then I found this article that describes what is disabled in Troubleshoot Mode... https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird#w_what-does-troubleshoot-mode-disable '''SOLUTION:''' Lo and behold, the answer lies in the default chrome/userChrome.css in the Profile folder. I commented out everything in userChrome.css, and that solved the problem – I now see the buttons in the message header as per the attached screenshot. Here's the default CSS code in userChrome.css, with the culprit commented out... #dateValueBox { display: none } /* #header-view-toolbox, #otherActionsBox { display: none !important; } */ #expandedHeaderRows { padding-top: 0px !important; }