Форум поддержки Firefox

My Windows 10 PC stopped working a few days & is disconnected & partially disassembled. I’m temporarily using an old Windows 7 PC which I brought up from the basement the following day. They both have Firefox on them.

I saved one bookmark on the Win7 PC today. It was there when I looked earlier. Now I went to use it again & it’s not there. But my Win10 bookmarks are there!!

How could that be?

To upload/import bookmarks into Firefox, the file extension of JSON is required. However from old crashed computer, I exported bookmarks in Excel. Please help. Thanks