Go to about:config
Set browser.tabs.useCache to true
I filed a bug that they make this the default: https://bugzilla.mozilla.org/show_bug.cgi?id=1615891
Because, why not? The feature was enabled then subsequently disabled in 2016. And for years I had to wait for getting off metro to read stuff in my tabs because Android decided to kill Firefox at some random point in time and Firefox would rather show Server Not Found page.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko