browser.tabs.executeScript() or something other operations processed just after browser.tabs.discard() break synchronization of tab's "discarded" attribute with actual status of the tab. Steps to reproduce: 1. Install the attached testcase addon as a temporary addon via about:debugging. 2. Start debugger for the addon and open the debugger for it. 3. Open new tab, and search something like "Firefox" with Google. 4. Open multiple tabs from the search result. 5. Right-click on a background tab and choose "Mark tab as loaded when unloaded" => "Unload and execute script". It just does: ---- browser.tabs.discard(tab.id); browser.tabs.executeScript(tab.id, { code: `browser.runtime.sendMessage({});`, runAt: 'document_start' }); ---- 6. Click the tab you executed the "Unload and execute script" command. Expected result(1): * At the step 5, the console says: tabs.onUpdated: XX, discarded=true discarded=true (0.5sec later) * At the step 6, the tab is discarded and start restoring. * At the step 6, the console says: tabs.onUpdated: XX, discarded=false Expected result(2): * At the step 5, the console says: tabs.onUpdated: XX, discarded=true tabs.onUpdated: XX, discarded=false discarded=false (0.5sec later) * At the step 6, the tab is not discarded. Actual result: * At the step 5, the console says: tabs.onUpdated: XX, discarded=true tabs.onUpdated: XX, discarded=false discarded=false (0.5sec later) * At the step 6, the tab is discarded and start restoring. Note that the log says the tab is already undiscarded but actually discarded - the "discarded" status reported via WebExtensions API is mismatched to the actual tab. This problem is originally reported at: https://github.com/piroor/treestyletab/issues/1903

Bug 1464992 is mentioned here: https://bugzilla.mozilla.org/show_bug.cgi?id=1378647#c39

I don't understand why there are two expected results for a single str, but I think it's just a copy/paste error since the second "expected" is the same as the "actual". Having said that, the actual results are now: tabs.onUpdated: 8, discarded=true background.js:38:5 discarded=true background.js:33:3 tabs.onUpdated: 8, discarded=false As such, this was fixed by bug 1422588