Not to worry. There's a program you can buy called Mobile Master, which lets you synchronise various mobile phones with sundry mail clients. Problem solved.
Well, not quite. Right from the start I've been suffering all sorts of glitches with the data transfer. Some events that recur every fortnight in Thunderbird show up every week on the phone, others shift by a day sometimes, and the 'Other data' field in the Contacts gets strangely mangled when sent back from phone to PC. All that was insignificant compared to the way that the entire calendar got downloaded to the phone twice (most times; tantalisingly, it would work occasionally).
There are fairly frequent updates to Mobile Master, which I would install hopefully, and a few months after I'd bought the phone, suddenly the transfer started to work every time. Events would get sent down just the once, and the other misbehaviour I fixed by deleting the affected items in Thunderbird and recreating them. (No idea why this was necessary, as they were fine when I used to synchronise with an iPaq.)
I stopped updating Mobile Master then. The newer versions were mostly concerned with supporting the latest models of mobile phone. But then, a couple of weeks ago, I took leave of my senses and upgraded to 7.5.5.
It was mostly okay, if you weren't bothered about not having any calendar items on your phone any more.
I raised a bug report, which had as much effect as the one I'd raised about getting stuff sent down twice. As before, I got back an automated reply with the ambiguous message:
We have received the following message and will answer as soon as possible. Sorry, but we can no longer answer questions whether this or that phone is supported.
Had they written off my query as a question about support for the n85, or was that just a standard line added to every reply? As I've never received a further reply, the question will remain open.
Luckily I still had an earlier version of their installer on my hard drive, so I was able to go back to that. A bit too early, unfortunately, as I'm back to having two copies of the calendar again.
Some time soon we are promised Thunderbird 3.0 will be released. Unlike Thunderbird 2, this will have a calendar system in-built, instead of as one of two possible add-ins. Perhaps it will build up enough market share then to make Nokia think it's worth supporting. (A quick web search suggests that Thunderbird 2 only has 1.12% market share at the moment, so I might be whistling in the dark on this. On the other hand, Lotus Notes only has 1.72%.)
In the meantime, I wait for another update to Mobile Master, and remind myself more often not to bother updating something that already works perfectly well.