Showcase 0.9.5.8 works great with Firefox 10 (ESR) and 11 by just disabling its cache option Rated 5 out of 5 stars
After updating from Firefox 3.6.28 to 10 (ESR) I got many crashes (also for Firefox 11) and after a view hours I was so disappointed that I just restored my system from the hard drive images I made before.
But I read Firefox 3.6.28 is end-of-life and therefore I tried Firefox 10 (ESR) again and enabled one plugin after the other and located that Showcase caused these many crashes.
I asked Josep for help and he told me this is a Firefox bug, NOT a Showcase issue.
To make Showcase working fine with Firefox 10 (ESR) too, you just go to Showcase options, select the advanced tab and DISABLE the topmost cache option.
This is Firefox 10 (ESR):
This is the Firefox bug causing the crashes: