I'm not saying that they won't continue to do so, but there is no track record of that since this is FiiO's first device that needs firmware updates, and the X3 has yet to be fully released in all its markets. Companies often don't follow through with needed additional firmware fixes for electronic devices after they are on the market; they go on to focusing on their next product. Then again, if the X3 is successful, FiiO could decide to do a major revision of the firmware to improve the product even further. There's just no way for us to to know until it happens.
The current firmware isn't just for X3 as most of the core work will likely carry on to X5 as well, so there is incentive for FiiO to get it to work as best as it can. I won't expect a major structural change to the firmware at this point, more likely just UI improvement and bugfix. However, I also don't think they will ever be a bug-free X3. Not even Apple can make a totally bug-free iPod / iPhone. It is just a matter of how far the bugfix will go until the bugs are rare enough not to affect normal operation. That, I think, isn't unreasonable to expect.