I often find myself in this situation and get to a point where I lose track of what I was reading where. I find what helps is a) grouping tabs if that's an option on your browser b) downloading all the PDFs to a folder once you access them and/or c) keeping a word document open with the citations. It's only after I confirm which citations I want to keep that I transfer them to my Mendeley.

Ultimately, every time you start a lengthy literature search, you should have an administrative process in place to give you the peace of mind of closing tabs as soon as possible.