Ideally, I’d have liked to find a way to customise that too-clever-by-half message, but I couldn’t spot any obvious way of doing that (share one if you have it in the comments). However, you can tell Firefox not to offer the session restore option at all, which can be a reasonable trade-off if you don’t routinely have dozens of tabs open at once.
To make that change, enter
and click through the “Here be dragons” warning (another example of Firefox being a tad too clever). Use the filter to find the
flag, right click, and select Toggle to switch it off.