Hey there.
The Fire Department is still around, and thriving. There was nothing inherently wrong with the faction itself, and it wasn't an activity issue that caused lack of members on, it was lack of motivation. What most people didn't realize about the faction, is on SAMP we were very limited on an out-of-character perspective. No one was allowed to set player-owned properties or businesses on fire without permission from the player, paired with authorization from an Administrator. If we wanted to give the faction members fires to put out, it was always scripted by Admins, or Fire Officers and Chief Fire Officers. These were "artificial" fires, as they were made specifically to give faction members fires to put out. This meant the majority of the time, Fire Officers had to sacrifice genuine roleplay for their members to have something fun and enjoyable to do. It also made members reliant on Admins or FOs to be around, and willing to make a fire. Which was limited on where you could set a fire, because of the server rules.
Medical scenes limited us as well. The majority of the time we'd arrive on scene, people would just /acceptdeath over very small or minor injuries, to avoid either roleplaying the medical scenes out, or to avoid arrest. In other cases, we had people just blatantly say they were salty they lost the shootout, and wanted to skip everything entirely, so they'd just /acceptdeath then. This led to the same outcome: Admins or Fire Officers having to sacrifice their own time away from the roleplay to create scenes for faction members to work on.
The majority of our roleplay was heavily dependent upon /mes and /dos, with no real scripted system, so even if we were to simplify what was going on, often times players would get lost, and simply not know how to respond. Or we'd have people lie about their injuries to survive, and avoid losing clothing objects, even though via the /damages, we could see they received a head shot. We had no concrete script for damages, or vitals. It was reliant on a he-said-she said scenario.
We can't speak on everything that's going on in the background, but our goal is to have more scripts involved, to make the faction inclusive, and more interactive for all parties involved, regardless of medical knowledge or comprehension.
I'll leave @Genny to explain further on areas I did not touch upon.