I run Windows Task Manager all the time (to guide me in maintaining RAM usage below 80%). I tend to have 4-10 Firefox windows open; if careless, I allow enough open tabs (e.g., RAM usage) to slow or crash FireFox/Windows.
The ability to quickly save (~all tabs) and CLOSE a Firefox window, helps me avoid Firefox/Windows crashes.
On 6/24/2020, I discovered FoxyTab, which
allows me to quickly copy almost all
URLs (in the "current" window of FireFox)
to the clipboard, and then paste them into a file (to save "where I'm at" in the Firefox window). (Optionally, I usually include Webpage-Title for each tab.)
With FoxyTab, I can even do this fairly quickly for ALL my Firefox windows.
Afterward, I can winnow/add-to/rearrange the "tabs" within that file.
FoxyLink AddOn allows me to later restore (~all) those tabs within a Firefox window,
often after (to free-up RAM) I reboot FireFox and/or Windows.
I have found both FoxyTab and FoxyLink to be highly reliable, and reasonably straightforward to use, though I often have to review FoxyLink usage instructions; the two AddOns have slightly different orientations.