docs: update firefox storage api docs

[vernak2539]

I just went through the process of publishing my extension to the Firefox add-on website. This is something I had problems figuring out from both the plasmo docs and Firefox's/MDN's docs. So, I figured I'd document it here based on what I found and had to do.

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 23, 2024 0:26am

[vernak2539]

@louisgv any chance you could have a look?

[louisgv]

Thanks for the nudge @vernak2539 !