A bookmarked page opens in a narrow screen on the left not the main page
When I click on the bookmark, a new pane opens on the left of the screen with the bookmarked page. This pane is too narrow to read or use.
Alla svar (1)
Right-click that bookmark and select Properties, then un-check the preference for Load this bookmark in the sidebar, and then click Save.
Using JavaScript buttons on web pages for "Bookmark this Page" causes that preference to be selected when saving the bookmark in that manner. So, don't use those buttons, use the "Bookmark this Page" feature that is built into Firefox instead.