The Firefox Safe Mode is a great way to compare a default Firefox installation with your current one. It can be used to analyze problems that are encountered in Firefox. The easiest way to start the Firefox Safe Mode is by closing all instances of Firefox, pressing Windows R, entering firefox -safe-mode and hitting enter. A box entitled Firefox Safe Mode appears.
This box not only allows to start Firefox in Safe Mode by clicking on the button Continue in Safe Mode but provides access to options to reset certain Firefox elements. Firefox will not load any themes, extensions and toolbar customizations in Safe Mode which is great if you are experiencing difficulties and want to quickly compare if they are related to extensions or themes.
The Safe Mode can also be used to disable all add-ons temporarily to be able to start Firefox normally if an add-on causes startup problems. The four other options available in that menu are permanent and a good solution if you want to reset the options to their default values: (Note this is only going to happen if the button Make Changes and Restart is clicked)
Reset toolbars and controls - any toolbar customizations will be removed
Reset Bookmarks to Firefox defaults - all current bookmarks get removed and the default Firefox bookmarks are added.
Reset all user preferences to Firefox defaults - All firefox options will be reset to their default values.
Restore default search engines - All default search engines that have been deleted will be added again.
It sometimes makes more sense to create a new Firefox profile instead which is also using a default setup at first startup.