You should use github, and host your project on there, it makes it much easier to share your project with others. It's likely none of these files you posted here are relevant. If you dig through your project, you'll find FirstMod somewhere (besides a class name), I don't think the log would just change it to capitals if it were actually lowercase. Double check all your java files, and any jsons you may have created by hand.
Also, you still did not share the full log.
I would wager the error is more related to your modid having capital letters in it
And also, don't post single lines from a log, especially when you're a novice, the error isn't always where you think it is based on a single line of the log. Post the entire log, use a paste site like github gist or something.