Improve Firefox automatic update CSS inconsistency

[marcustyphoon]

Concept

As noted in discussion #1353, when Firefox pulls a new extension update, changes to scripts' static stylesheets aren't applied until one opens a new tab (while changes to script code and dynamic stylesheets are, potentially leading to temporary inconsistent behavior).

Not sure we actually have a method to improve this (obviously besides using only style elements, which does have some significant advantages), but even if that's the case it seems worth having an issue filed.

[marcustyphoon]

You wanna know what's really scaring me? The link-element-pointing-to-a-css-file-in-the-extension-package method, when loaded via web-ext run, doesn't even show changes when I press refresh on the page. I have to command-shift-R. Surely that can't reflect the real update experience... right?