This looks definitely like a duplicate of Bug 1429838, which is due to the fact that the absolute paths to the extension xpi is part of the details included in the profile (in extensions.json to be precise). In particular - based on comment 0 it looks that the path to the profile on device B is different from the path to the profile on device A, and so: - on Device B, running Firefox from the profile copied will hit Bug 1429838, in particular because the extensions will not be available at the absolute path listed in the extensions.json file part of the Firefox profile copied from Device B - on Device A, when the profile is copied back from Device B but in a different path, the extension listed extensions.json file will be available at their original absolute path on Device A (unless removed from that path, e.g. if that profile has been moved instead of copied then it wouldn't work on Device A neither) - based on comment 4, the Firefox profile seems to be stored in the same absolute path on both Device A and Device B, and so it is expected to don't be able to reproduce the issue described in comment 0 (because it would not be recreating the condition needed to hit Bug 1429838). The XML error on the extension newTab.xhtml is also something that Bug 1429838 would be able to trigger (e.g. because the page, or other resources needed by that page if the page have been able to be restored by the session restore, would be failing to be loaded because the extension xpi file is not at the path that Firefox expects and so the extension assets would not be reachable anymore).
Bug 1765741 Comment 5 Edit History
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This looks definitely like a duplicate of Bug 1429838, which is due to the fact that the absolute paths to the extension xpi is part of the details included in the profile (in extensions.json to be precise). In particular - based on comment 0 it looks that the path to the profile on device B is different from the path to the profile on device A, and so: - on Device B, running Firefox from the profile copied will hit Bug 1429838, in particular because the extensions will not be available at the absolute path listed in the extensions.json file part of the Firefox profile copied from Device B - on Device A, when the profile is copied back from Device B but in a different path, the extension listed extensions.json file will be available at their original absolute path on Device A (unless removed from that path, e.g. if that profile has been moved instead of copied then it wouldn't work on Device A neither) - based on comment 4, the Firefox profile seems to be stored in the same absolute path on both Device A and Device B, and so it is expected to don't be able to reproduce the issue described in comment 0 (because it would not be recreating the condition needed to hit Bug 1429838). The XML error on the extension newTab.xhtml is also something that Bug 1429838 would be able to trigger (e.g. because the page, or other resources needed by that page if the page have been able to be restored by the session restore, would be failing to be loaded because the extension xpi file is not at the path that Firefox expects and so the extension assets would not be reachable anymore). Does this correspond with what you can also verify locally? Let me know if there are other elements that doesn't seem to match Bug 1429838 and I'll re-open this bug.