I am running firefox 8.0nb2 (from pkgsrc head) on NetBSD/i386 5.1_STABLE (head of netbsd-5), and have been running various firefox 4-8 versions along the way, with relatively little trouble. I do get the 'failed to create drawable' message. I also find that depending on what kinds of web pages I look at firefox gets huge (1G) but with a smaller RSS, and then it gets very slow, so I kill and restart it. I use this on machines with X server from pkgsrc and from base, on boring intel 845 or 915 chipsets. So I wonder if there is some fancier X option in your card and firefox is trying to use it and failing, and if turning off things in your x config would help.
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