Whether you open in https or http is down to the bookmark and if the site supports it - perhaps AAM has changed recently to require https to edit? The admin will know if he has. Other than that, have you installed any add-ons on Firefox recently? They may be causing the issue.
If the only solution is to change all your bookmarks, try exporting the bookmarks first, editing them in notepad then doing a "Find and Replace" for http:// to https:// - then you can import them back in and it should have updated your bookmarks in bulk.