Someone has approved your request or your approved theirs and now this nervous moment of sending the first message... I suspect that this is a rudiment from ordinary dating sites, where the first message is actually unsolicited and a bad first message can lead to rejection. In case of firedating.me, relax, there is an approved request already, so both of you find each other interesting. If your request was approved, they looked at your profile and based on that decided that you are interesting enough to actually talk. If you approved their request, there was a reason for that too. So the first step has already been made. Thus, just relax and chat.
Ok, so how to find a topic? I personally like to discuss something I find interesting about them. Usually if the profile is reasonably filled in, there are plenty of topics like that. They may have an interesting hobby, which I would like to try. Or we may share a view. Or they may have a different view. Or there is something interesting on their picture. E.g. where was it taken? In the end, something caught your eye about them in the first place, because there is an approved request already, so you could ask about that.
Although the site is much larger than I originally expected (~1450 people), the users are still very dispersed geographically. So if you look to communicate "offline" (especially "romantic"), very likely you see few profiles. It is unfortunately so. I myself was in this situation for quite a long time too. You can obviously spread the word locally to change this (TODO: add link to post with ways how users can help me with the site) or make your criteria broader.
I am also working on a new feature - geo-based search - which will allow to see more people nearby (including from other cities).
You could try choosing large cities close to you (especially if you live in a smaller one). Perhaps there is someone there. Or you can ignore location altogether. Chatting with nice people, who are abroad, still better than not chatting with anyone. Also you never know where it can lead to ;)
You can switch to friendship. WARNING: please don't hit on people you match with this way - respect their decision to look for friends. While I don't say that you two are forbidden from having anything romantic, please keep in mind that they have chosen to look for friendship for some reason. I believe that building romantic relationship without having friendship is tricky, so I suspect that this way might be even more productive - first become friends and then you will see how it goes :)
Finally the most broad category is "friendship online". If you feel lonely and want to talk to someone - the best decision.
As a summary I suggest the following sequence of search parameters:
Romantic offline in your city
Romantic offline in large cities nearby
Friendship offline in your city
Friendship offline in large cities nearby
Friendship online (but don't hit romantically on them).
Obviously if you look for friends, you can omit all "romantic" steps above, but the principle holds. I personally went from "romantic offline" to "friendship online" and then "romantic online" out of curiosity (how people imagine "romantic online"). I think the latter worked more like "friendship online" for me - I did get to know wonderful people, but some people actually rejected me :)
Happy dating and let the FIRE be in your heart!