Recover Lost or Missing Bookmarks 1] Unable to set Firefox as the default browser.Firefox Unable to download or save files.Firefox unable to save usernames and passwords.Firefox unable to save changes to toolbars and window sizes.Unable to set Firefox as default browser:.In this guide we are covering these scenarios: It’s key to fixing all the problems in Firefox. Just before we start sharing the fixes, if nothing works, Refresh Firefox at the worst. If that doesn’t happen, check our guide which will help you fix these issues. Firefox offers a plethora of customization to personalize Firefox to what it suits you best. Can’t change a setting in Firefox? Can’t save the homepage in Firefox? Does it not remember passwords? Many a time when you try to change settings in Firefox, it won’t save.