This does not happen with Firefox 3.6 - Firefox 4 only. Could be related to the recent enabling of border-radius. The link is to a reduced testcase of an actual site where selects became invisible. Again apologies, this should probably be filed in Core - still that javascript bug with the template, but this bug seemed reasonably significant, and I have a bad habit of forgetting to file when I can do so from a computer with javascript.

Could you please attach a testcase?

To elaborate on the testcase, if you inspect the form element in Firebug, you can toggle off either the border-radius or the overflow-x rule to see the selects reappear. Toggle the rule on again, and they vanish.

I see two drop-down-lists in that testcase. What OS are you on? Mozilla/5.0 (X11; Linux x86_64; rv:2.0b7pre) Gecko/20100914 Firefox/4.0b7pre

Huh. I'd assumed it was generic. Go figure. Mozilla/5.0 (Windows NT 5.1; rv:2.0b7pre) Gecko/20100914 Firefox/4.0b7pre Just retested in a clean profile after a moment of panic. You're right though. Does not impact my OSX or Linux machines, even if I set a border: 1px solid black; just to turn off native styling.

Does this depend on whether graphics acceleration is turned on on Windows?

I set: layers.accelerate-all;false layers.accelerate-none;true And I still saw it - is there something else I should set? Also reproducible in Windows Vista in a test VM BTW.

OK. Next thing is probably finding a regression range....

Oh, also try setting mozilla.widget.render-mode to 0?

No difference between -1, 0 and 6 for render-mode

Adding -moz-border-radius in case the regression range carries me beyond the recent enabling of border-radius.

Problem is NOT visible in 2010-09-08 prior to the push of border-radius. It exists regardless of whether a page is using -moz-border-radius or border-radius which is not really a surprise I suppose. http://m8y.org/tmp/testcase197.xhtml (page using -moz-border-radius)

Problem appears between 2010-09-09 and 2010-09-10 Sooo, between: http://hg.mozilla.org/mozilla-central/rev/8e0fce7d5b49 and http://hg.mozilla.org/mozilla-central/rev/79d0beec27b5 And just as a guess, the most recent border-radius fixes? :)

Yes, I bet.

Does the problem go away if you use -moz-appearance:none on the select?

Hm. Thought I tried that. But in any case, can't check right now, all my windows machines are at work. If anyone over there has a windows box to test on... Otherwise, Monday. :)

Probably bug 595842, bug 596205, and bug 596271 (this bug) are related.