Add Firefixes

Validation tests on Firefox works better if the element is attached to the
document.

Gecko wont implement validation logic for _tooLong_ validations since they don't
believe that people can use JavaScript to bypass _maxlength_ attribute. [1]

So, I confess, I did browser sniffing to detect Gecko and perform tests against
too long with ```!isGecko``` when required.

Since _type="number"_ will be shipped with Gecko v29 (aurora channel at this
time), we'll leave the 6 related tests failing on Firefox.

[1] https://developer.mozilla.org/en-US/docs/Web/API/ValidityState