fwiw, if you're on a slow system and you can get the mouse outside the button when the menu starts to fade it, the menu is briefly usable.

I could reproduce this with Iceweasel 38.2.1 on Debian stretch, no add-ons.

Checking Archive.org's snapshot of store.steampowered.com, the problem was introduced between the last snapshot on September 8 ( https://web.archive.org/web/20150908194136/http://store.steampowered.com/ ) and the first snapshot on September 9 ( https://web.archive.org/web/20150909032714/http://store.steampowered.com )

The corresponding Mozilla bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1205244

Same here with firefox 41.0.2 on ubuntu 15.04

This bug is on all operations systems. My bug report:
https://bugzilla.mozilla.org/show_bug.cgi?id=1205027
-1st problem is scroll down and FF not load recommendations, sometimes load one recommendations list
-2nd problem is the drop-down lists for Games, Software, Hardware, and For You disappear when you try to click one of the items

Hi,
the menu problem is caused by some JavaScript code relying on a non-standard IE-originated DOM extension - it uses event.toElement instead of event.relatedTarget. I'll copy from a comment a Firefox developer makes in https://bugzilla.mozilla.org/show_bug.cgi?id=1205244 :

The JS file in question is:
http://store.akamai.steamstatic.com/public/shared/javascript/shared_global.js

That file does actually have some usages of "relatedTarget", but it uses
non-standard "toElement" in the crucial place for these menus to work, which is
this chunk of "BindAutoFlyoutEvents()":

   $J(document).on('mouseleave.Flyout', '.flyout_tab', function(e) {
   [...]
           if ( !$Content.length || $Content.data('flyout-event-running') || bResponsiveSlidedownMenu ||
                   $Content.is( e.toElement ) || $J.contains( $Content[0], e.toElement ) )
                   return;

Using event.relatedTarget instead should work.

Same bug reported at https://github.com/webcompat/web-bugs/issues/1886#issuecomment-153962858.

Problem is resolved with menus disappear, when you install Greasemonkey and this script:
https://greasyfork.org/pl/scripts/13121-fix-drop-down-menus-on-steam-store
Not help with not loaded recommendations on Steam Store in Firefox.
Maybe little userscript to fix this problems? PLS

Userscripts are cool for power users, but ideally someone at Valve can fix this for everyone.

Valve has fixed this internally, and it should be fixed on public sites sometime next week, per https://bugzilla.mozilla.org/show_bug.cgi?id=1205244#c19

The weak nexus to Steam-for-Linux aside, this has been fixed.

It's happening again!

Firefox 58.0.1