Environment: Windows 10 22H2, Thunderbird 128.0.1esr 64-bit on two different PC's, "A" and "B". Scenario and steps to reproduce: (1) On "A", installed extension "remove duplicate messages" (version compatible with Tbird 128). Works fine. Profile on this PC is D:\Alice\Thunderbird\Main. (2) Completely get out of Thunderbird on "A". (3) On "B", launch Thunderbird accessing the same profile residing on "A" as mounted drive, "Q:\Alice\Thunderbird\Main". Mail, calendar, etc work fine. However, the extension is not available (No "Remove Duplicates" on context menu or Tools menu). On Tools :: Extensions and Themes, extension shows up with a blank (empty) icon. (4) On "B", disable the extension, close the "Extensions and Themes" tab and completely get out of Thunderbird. Re-launch Thunderbird, go to Tools :: Extensions and Themes, and the icon has reappeared. After re-enabling the extension, it is active and works flawlessly. (5) Then, completely get out of Thunderbird on "B". (6) On "A", launch Thunderbird. It now exhibits the same behaviour for the extension: it is not available, and the icon is blank. Performing the same steps as in step (4) above restores the extension to its working state. And, you have probably guessed by now, it is no longer working on "B" ! Just to make sure this is not specific to this particular extension ("remove duplicate messages"), I installed another extension ("Quicktext"), and it behaves exactly the same (either singly, or with "remove duplicate messages" still installed). So, it seems that you cannot use extensions from the same profile accessed via two different paths, one PC accessing it via a local drive and another accessing it via a mounted drive. I want to re-interate, before someone asks, that Thunderbird is completely down on one PC before I try to access the profile on the other PC. Has anyone seen this behaviour on Tbird 128 ? Thanks.