Qualifications
This may be a complete shock to you, but most fire investigator jobs can be found in the local fire department. Because 75% of fire investigators are employed by local government agencies, the qualifications you need will be determined locally.
In some towns, all you'll need is a high school diploma and some experience working on the local fire crew (with a higher rank to match). Others will expect you to go take classes in fire science, fire safety, and fire writing (we made one of those up). A balance of field experience and classroom study will make you a prime candidate in most towns.
If the A-T-F is more your cup of T-E-A, then you'll need at least a B-S-D. That's a bachelor's of science degree. That means a four-year education in addition to all the experience you have fighting fires.
Working for the Federal government has its perks, but that's because they're not going to let just any weirdo with an explosives obsession walk in and investigate crime scenes. They expect you to at least be a weirdo with a degree (source).
Other, more specialized industries will have their own particular set of skills. Insurance companies need someone who not only investigates the causes of fires, but also determines the general cost to replace, repair, or rebuild. Lab techs who analyze chemical components to figure out which kind of boom we're talking about will need those more "legitimate" science degrees.
And if you've got enough experience researching those great balls of fire, you might even get a job helping safety designers prevent them before they happen. Just don't tell Jerry Lee Lewis, he might take that personally.