This article lists keyboard shortcuts in Mozilla Firefox. The shortcuts only work if they are not used by the desktop environment or window manager. If you have enabled Emacs-style text editing shortcuts in GNOME, they will also work in Firefox. When an Emacs text editing shortcut conflicts with the default shortcuts (as occurs with Ctrl+K), the Emacs shortcut will take precedence if focus is inside a text box (which would include the address bar and search bar). In such cases you should use the alternate keyboard shortcut if one is listed below.
These shortcuts require the tab bar to be "in focus." Currently, the only way to do this is to focus an adjacent object and "tab into" the tab bar, for instance, by hitting Ctrl + L to focus the address bar, and then Shift + Tab repeatedly until the current tab gets a dotted rectangle.