Tabs open within other tabs. There is no protect tab option to prevent this. We have two separate library memberships and Firefox now combines them because they both use Overdrive, or, perhaps because there is no way to establish that tabs opened from bookmarks must always open in a new tab (which was my default for all situations). When I close Firefox, it no longer asks if I want to save all the tabs that are open and when I restart, many are gone, even the first tab which is always open to my gmail account, disappears. I am trying to use Multi-account Containers to resolve some of the problems, but it is a pain to use. One has to open a new tab via Containers, enter the address and then select that this site is to only be opened in that container group, as opposed to already being at the site and being able to right click and add it to the container. Tab-Mix Plus was critically central to my browsing in Firefox, and I am frustrated and at a loss without it.