Welcome to the forum
Yes, everyone can make their own repo. The post you linked is about outdated, though, so you may like to use the official guide from the F-Droid site
Basically you just need
fdroidserver. When you have this set up, you can create a repo like shown in the guide and then upload it to somewhere you want. You don’t have to set up your own web server but also can use any Git service like GitLab or GitHub to publish your repo. If you have access to some FTP/SSH storage, you can publish it there too. Or you can use Amazon S3. There are many possibilites, but for the start a Git repo is enough, I think.
Yes, many people would like to use Firefox, I think. It would be better if the repo is created directly by the developer Mozilla, but since they don’t do it, the community has to find their own ways.
I’m personally against including repos that include non-free software and Firefox includes some Google Play dependencies iirc. Instead you can add your repo to the list of known repositories and we can think about linking this list from the client.