The other evening it was raining but I wanted to have a play with something outside, so I decided to see how a firestarter (the idea for which I came up with the idea for over a decade ago) stood up to rain. It's my drinking straw firelighter. Basically, it's a length of drinking straw (try BK or McD because they have a good diameter) stuffed with a cotton wool/vaseline mix and sealed either end with a dot of hot glue. The straws can be cut to the desired length (I had 4 inch lengths that fitted in a survival kit).
First, I left it out in the rain for a few minutes. Then, I made a split in the straw halfway along it and fluffed up the cotton wool. Then, using a firesteel I lit it - still in the rain!
The straw and contents burned fiercely for about 4 - 5 minutes, seemingly unaffected by the rain.
As a side note - I had made the straw that I burned a good 4 years ago, so these things last a long time and don't leak or deteriorate over time.
I find there a good bit of kit to keep in reserve for when you can't find decent tinder or need to get a fire going fast.