Plug-ins and extensions are stored in three different places:
Hard Drive -> Library -> Internet Plug-Ins
Home Folder -> Library -> Internet Plug-Ins
Home Folder -> Library -> Safari -> Extensions
If the problem started after installing a certain plug-in or extension, find it in those folders and remove it.
Otherwise, try removing these one at a time until it works:
Home Folder -> Library -> Caches -> com.apple.Safari
Home Folder -> Library -> Caches -> Metadata -> Safari
Home Folder -> Library -> Safari
Home Folder -> Library -> Preferences -> com.apple.Safari.plist