I definitely find them useful as units for deterring a com push, although I worry that usefulness is probably mitigated by priority targeting mods.
If they were just a little faster, they'd be a good counter to T4. Not saying that we SHOULD do that, but... I wouldn't complain...
I think firebeetles have a limited role as eco damage drop units, although this isn't particularly sexy as T1 arty for other factions does it better.
Its not a terrible option though, as they don't leave reclaimable wreckage usually, so if you get a mex, the mex is gone and they don't even get the wreckage of your beetles to help rebuild with.
If Cybran transport capacity wasn't so low they might be seen as slightly less useless.
I think they're no longer in the "completely useless for any particular purpose" category, and are now in the "needs reasonable minor tweaks to really shine" category.
Dropping them to say 160 mass might be enough to change people's perceptions of them. I don't think they need anything like a major rework at this point.
In Leage of Legends, the devs have figured out a neat trick...
When they deploy a new character, the character is always moderately OP. This is to get people to take notice of them.
I think the firebeetle needs that. They need a moment where they're OP enough that people acknowledge that they exist and serve a purpose.
After that you can nerf them back into a reasonable position, and people remember how good they were and see how they can be useful and everyone will be happy.