If you mean sort them in alphabetical order, use Bookmarks > Show All Bookmarks (ctl+shft+o), then the menu at the top Views > Sort >>

Note that using "Views -> Sort" only sorts the bookmarks temporarily for viewing purposes in the Bookmarks Manager (Library) similar to clicking the column header to sort by a column. Once you close the Library then this sorting is lost.

The way to sort bookmarks is by right-click a folder and selecting "Sort By Name".

• https://support.mozilla.org/en-US/kb/sort-bookmarks-quickly-find-ones-you-want