The support for that I've been working on for Mozilla is now turned on for Aurora 28 and Nightly builds. If you're interested in using here are a few things you should know:

• The current support should first ship in Firefox 28 (scheduled to release in March), barring any issues that would cause us to disable it.
• As of yet, locale specific user input is not supported. In other words user input that contains non-ASCII digits or a decimal separator other than the decimal point (for example, the comma) will not be accepted. If this turns out to be unacceptable then we may have to turn off support for Firefox 28, or else add a hack to at least accept comma. Ehsan has just finished battling to get ICU integrated into gecko's libXUL, so I should be able to fix this properly for Firefox 29.
• Unprivileged content is not currently allowed to access the CSS pseudo-elements that would let it style the different internal parts of the element (the text field, spinner and spin buttons). The only direct control content has over the styling of the internals is that it can remove the spinner by setting -moz-appearance:textfield; on the input. (The logic and behavior that defines will still apply, just without those buttons.)

If you test the new support and find any bugs please report them, being sure to add ":jwatt" to the CC field and "" to the Summary field of the report.