Turn off middle click close tab function
While using firefox with mac trackpad or magic mouse it's really easy to accidentally middle click to a tab and close it. It's really frustrating to loose a tab. I guess it wouldn't be so hard to put an option to disable that function. That would make life so much easier. And there are LOTS of requests for the same thing in this site many of them more than years old.
Wšykne wótegrona (3)
There is no way to remove the middle-click action to close a tab. You can easily undo an accidentally closed tab via "History -> Recently Closed Tabs".
If the page forbids caching or clears fields on restore, then unfortunately you will lose data.
It is possible to inject a script into the page to prevent it from closing immediately. You may occasionally have seen a prompt about whether you really want to leave the page. I don't know whether there are any add-ons you can use to inject such a script specifically into pages with forms, or just when you want it there, but that might help until someone comes up with something better.
Or you can use an add-on that saves form content as you type. I don't use these myself, but these are two that have been around for quite a while:
Musyśo se pla swójogo konta pśizjawiś, aby na pśinoski wótegronił. Pšosym stajśo pšašanje, jolic hyšći njamaśo wužywaŕske konto.