Having the Add-ons Manager page freeze, would entice me into uninstalling the extension immediately and doing more research. You did not name the extension, and of course, the add-ons url to make it easier for many people to look at the extension -- more eyes. Not all extensions, in fact very few, get added automatically to the add-ons bar. You may find options to indicate where it's presences should be made known, or you may have to move a toolbar button from Customize ("Alt+V">Toolbars>Customize), or it may be in any of the menus or context menus, sidebar, or Library List among other possibilities.