You would think "Phoenix" would be a word you could use, but it didn't fly, and neither did "Firebird", and then even "Firefox" took licensing agreements because a EU trademark turned up later. The Diamond "Viper" video card name had to be licensed from a car alarm company. A EU or US name has a trademark infringement if it can be put in a category (numbered above) where there is a previous trade use.
Are trademarks valid also for categories which are out of business for trademark holder? I mean - can Disney really protect "piglet" for some electronic gadget?
Disney does business in electronic gadgets, and they have a trademark in that business category.
Btw Honda Firebird is also motorbike and Dodge Viper is a car. I'm not affraid that Disney will go over us with USB gadget (as far as we won't misuse Disney's piglet images).
Viper the car is in a different category than computer peripherals, but Viper the car alarm was found to be in the same electronics category as a video card. This is an example of how different products will be found to occupy the same trademark category.
Firebird the vehicle is a different product class than software, so using this name for something completely different would be fine if there was no trademark in the other area. Firebird also ended up being the same name as database software though, so the name was already taken in the same category. Thunderbird for an email reader doesn't infringe on Thunderbird the car.
Firefox FAQ:
Why did you choose Firebird in the first place?
With the assistance of attorneys we assessed other uses before selecting Firebird and did not consider these uses to be in the same domain as our web browser. It is very difficult to come up with a good name that is not also used by some other company or project, so choosing and verifying the availability of a name is a complicated process that involves many difficult judgment calls. If you have a small project, likely a large monolithic corporation won't come after you. That is, unless they are part of a large copyright and trademark cartel that enforces the status quo of media consumption and international limitation of free commerce, with lots of lawyers, and they don't like what you are doing.