While taking a closer look at the Twitter CSS I noticed .twtr-widget uses position: relative which might be the root of the problem. I think overriding this to the default value: static might solve the issue. I am not sure however because I haven't been able to test this out. Neither Firefox 12 or Chrome on both Windows and Linux, logged in or not, gave issues like in the screenshot. What are your thoughts on this?
I think it's possible that the ones showing what I saw in the screenshot may have had some custom config/plugins interfering... but other PC/FF12 users were having a different issue: no twitter content displaying in the tab.
position:relative does sound like a potential culprit. I played around on a test wiki, and found that even if you put the code directly below tabber, there were still issues. I think tabber is messing with positioning.
Well, i'd still really like to know what made twitter appear at the top of the page for some people (cuz that's a very ugly bug), but if we can test on a few other PC/FF12 installations and not reproduce it, we can probably safely say it's not bringing the end of the world.
So it is safe to re-introduce it? I tried all other positioning methods: static, inherit, absolute and fixed. All of them made the widget disappear completely, even on blank pages with only the widget in it.
About the cached version of testing CSS: I don't know, I did all the CSS modifications in my user-specific config before rolling it out wiki-wide. There were no modifications made to the twitter widget since a very long time.
The twitter widget is created by twitter itself, right? As it is imported from their domain.
The main page featured gallery slider <gallery type="slider">. Other wiki users on our IRC channel who recently updated to FF13 have the same issue. It happens on both Windows and Linux, on every wiki I visited with such a slider on the main page. This includes sliders not featured on the main page and preview windows.