Firefox prevents Open in Tabs from opening more than one tab

I'm not sure how recent a regression this is, but it's either been a problem for a while or they introduced it recently.

Theoretically it should work fine because we call `window.open` under a `click` event, but we may need to start asking the background page to open the tabs for us to prevent this sort of thing from being an issue

Leaving this unpatched for now just because maybe it's a me-issue

Start by tracing the Open in Tabs click handler and its window.open calls in Firefox, then compare that path with the suggested background-page approach. Reproduce the issue in Firefox and verify that Open in Tabs opens every selected tab rather than only one.

Written by the indexing model from the issue text.