Protect Specific Tabs From Auto-Unload от Steven Saus

While tab unloading is often useful, there are times you do not want a specific tab to ever unload.

Firefox already has internal rules that generally work well, but sometimes they miss tabs you care about. This extension examines open tabs, matches them against user-defined rules, and marks matching tabs so they are not eligible for unloading.

You can verify the effect using about:unloads.