When I store basil in the fridge, sometimes it gets brown spots. Is it still usable? Is that valid also for Thai basil? Throwing it all away after 2-3 days seems such a waste. Also, how can I prevent this from happening?
This is actually precisely the reason why it's not recommended to refrigerate fresh basil leaves.
I would not recommend consuming basil that has turned brown/black, especially if it is "slimy" to the touch. Even though a few brown spots are probably safe, it will be bitter and, well, slimy. Throw it away - and consider using some of the storage methods discussed in the link above.
If it's only been a few days, there's no sliminess or weird texture, and the smell is still fresh and basil-like with no hints of unpleasant decomposition, then what I've found is it's just a matter of presentation. It won't be very good for garnish, but if all you need out of it is its flavor, then it's fine. Blended into a puree or simmered in a sauce at the last minute, it still tastes fine to me. Even just by themselves, the mottled leaves have tasted fine, just a little rougher texture. So it's not necessarily a complete waste.
As long as they are NOT slimy and well cleaned use them. I blend in food processor with fresh garlic, olive oil and a bit of good Parma just so it sticks. Take a large tray or cookie sheet, cover with waxed paper drop by large spoonfuls (they spread a bit) and freeze. When frozen take off waxed paper and freeze in ZIP lock bags. You have a good beginning pesto for use in soup, stew/goulash pasta or risotto. The sky is the limit. I make this frequently in summer and again winter when I run out!!