Click into the search bar. Enter the first characters of the term that you want to delete; use the up-/down-arrow if necessary. Select the term and press the Delete key. Source: c’t 2009, #23, page 166.
Enter about:config in the address bar. Search for browser.tabs.insertRelatedAfterCurrent The value true (default) means new a new tab will be inserted directly after/right to the original tab, instead at the end. In order to force all links that open in a new window to load in a background tab, set browser.tabs.loadDivertedInBackground to true.
Enter about:config in the address bar. Search for browser.tabs.closeButtons Possible values are: 0: Only the currently active tab displays a close button. 1: Each tab has its own close button (Default). 2: Tabs without close button, a tab can only be closed via context menu or Ctrl+W. 3: Only one button (on the right) for… »»»
Disclaimer: This information is based on Firefox 4, now Firefox 28 is the current version, so maybe it’s obsolete… Enter about:config in the address bar. Search for gfx.font_rendering.cleartype_params.rendering_mode. Possible values are: -1: Firefox default 0: default 1: aliased 2: GDI Classic 3: GDI Natural 4: Natural 5: Natural Symmetric
Open Mozilla Firefox and enter about:config into the address bar, accept the warning. Search for browser.tabs.onTop and set it to False.