I was looking for an extension that does exactly this, and despite the reviews saying the extension only works if you turn it off and on again every time you reload a page, I installed it anyway. It did indeed need to be reactivated for each tab I opened or reloaded, but I didn't mind the extra step.

But a few weeks ago, I noticed that after a recent Firefox update I no longer need to manually activate the extension for it to work. It now works just as designed, and I have automatic smart quotes in every tab.

Now that the extension works as designed, I have no qualms giving it 5 stars!

Finally! I was so tired of Tumblr using straight quotes, but with this, i don't have to manually convert them anymore

This has been my constant frustration with writing on the web. I'm so glad I found this.