One of the killer features to me for using Opera was a feature to browse with images turned off (cached images only) and a "show image" option on the right-click menu: coupled with extreme caching, it allowed to have a very tolerable browsing experience on very slow networks (mobile GRPS, etc).
Firefox misimplemented this feature by missing its point -- its "view image" opened the missing image in a new page -- instead of just loading it in its place in the page, and I'm not sure it used caching as good as well.
Firefox misimplemented this feature by missing its point -- its "view image" opened the missing image in a new page -- instead of just loading it in its place in the page, and I'm not sure it used caching as good as well.