Update to firefox 3.5.5. This is a stability/bugfix update.

pkgsrc changes:
- assign devel/xulrunner maintainership to tnn@
- mozilla-common.mk: work around gcc __thread support misdetection on NetBSD
- separate distinfo related stuff into dist.mk for sharing with nss & nspr

"topcrash" bugs fixed:
468562 "ASSERTION: Inserting multiple children without flushing"
521750 Put a runtime NS_IsMainThread check in nsCycleCollector::Suspect2 ...
524462 startup crash [@ gfxWindowsFontGroup::WhichFontSupportsChar(nsTAr ...
525326 Crashes in gif decoder [@ xul.dll@0x348945][@ xul.dll@0x348864][@ ...
525276 crashes [@ nsDocument::RegisterNamedItems(nsIContent*)]

(tnn)