Jeff Overcash originally made it clear, that IBX doesn't officially support Firebird, and he has no intention to implement it. Many point at IBObjects and now DBExpress for Firebird support, so why use IBX? Here's a reason:
IBX supports Interbase 6.0, which is basically equivalent to Firebird 1.0. Firebird 2.1 supports the Firebird 1.0 API by providing a compatible gds32.dll, so Firebird 2.1 also works with IBX. IBX is part of Delphi, and has been part of Delphi for a long time, meaning that when there is a new Delphi version, IBX will most likely be included, too. This gives IBX a big advantage over IBObjects and others. I assume that IBX will still be around in 5-10 years. In case something should break with Firebird, the source code of IBX can be modified to fit.
Does this make IBX the best choice for doing Firebird with Delphi? No. But as long as it works well, there will be no need to switch, and I guess that a lot of people start using IBX simply because it is included with Delphi.