I noticed that add-on download progress doorhangers keep changing their width. Reason: Bug 570012 (changeset http://hg.mozilla.org/mozilla-central/rev/4294a3a4edf3) introduced a new urlbarBindings.xml binding and browser.css styles, among them this one: .addon-progress-description { width: 350px; max-width: 350px; } However we use our very own notification.xml which doesn't have the above (yet), and we don't have the above style rule. Note: Keep Modern and Mac in mind.

https://hg.mozilla.org/mozilla-central/rev/d8e2600e9aa3 https://hg.mozilla.org/mozilla-central/rev/4ad5047763c0 https://hg.mozilla.org/mozilla-central/rev/4a04ca5ed7d3