FF 66.0.1 Mac OS 10.12.6 Sierra Tried these solutions: 1) Restored browser settings; 2) About:config solutions (https://support.mozilla.org/en-US/questions/1226151;) 3) Only choice in preferences is open links in tabs instead of new windows, so tried with checkmark and without; 3) Reinstalled Firefox; 4) Upgraded Firefox; I am a web developer. I check my work all day by viewing in a browser. I have screens full of individual pages that I tediously have to clear away. The goal would be to get links to open in one tab. Before the last two FF updates, I was able to use the about:config solutions at the link above to fix the problem. But when I updated to the last version before the current, that solution no longer worked.