Hmmm, which mail site is this? One possibility is that you had a tab-related extension that automatically opened regular links in a new tab, and that the extension was disabled because it was incompatible with the latest version. To check for disabled add-ons, visit the Tools > Add-ons dialog. As a short term workaround: you probably know you can force a link to open in a new tab by holding Ctrl when you click the link, or by using the "middle" mouse button (often the scroll wheel). I find that quicker than right-clicking the link and choosing Open in new tab.