Some sites resist critical browsing. For example, if you try to enter the website of the Vlaams Belang, a right-wing Flemish political party, it will automatically launch in a new page. Regrettably, some sites which in fact deserve the most scrutiny have frame breaks implemented in their source.
Firefox bookmarks do not open in a new page. Due to the way it handles Javascript, the "Favorite" button produces a bookmark which will automatically open in a side panel. You can manually de-select this option once the bookmark is saved, or use Ctrl+D or Apple+D on your keyboard to override this setting.
This browser is made up of HTML pages using frames, a stylesheet, php and some Javascript. If you want to see how it is done download the files here.
Please send questions, comments and report bugs to: info at geuzen.org