I had a whole bunch of tabs that had been open for quite some time on my browser that I was planning to go through. I have my browser set to restore tabs / previous session each time I open so all the previous tabs remain there, and have had no problems for a long time. However the last time I tried to close Firefox it gave a warning saying I was trying to close x amount of tabs. I thought it a bit strange considering it hadn't asked me this before when closing but didn't think anything of it and closed the browser, only to open it next time to find all my open tabs had gone rather than being restored as normal.

I hadn't knowingly changed any of my settings and its still set to 'Restore previous session', and I can't work out a) why it hasn't restored it and b) what changed to prompt it to ask me if I wanted to close my tabs and then remove them rather than what it has always done!