I wouldn?t worry about getting firmware updates for my unbranded Nokia phone. When a firmware is available, Nokia releases the ROMs for every product code and makes them available online (if you know how to get them). Usually I use Nokia Care Suite to find out my phone?s product code and then download and flash the new ROM into it. This will of course wipe everything on the phone, but I can always have backups of everything and start afresh with the new firmware.
Nokia Care Suite is not publicly available to end users, but if you search online there are threads about how to download it and update it so that end users can make use of it. Just be careful when doing so because there is risk to end up downloaded harmful software.
By the way, I would still prefer to use this method to update my phone even if the update is pushed OTA. Just like formatting your PC and starting clean with a new system (if you have the ISO image), I like to have the new ROM image and do the same with my windows phone