Import Thunderbird helper module only in Thunderbird

This fix saves about 60k for non-Thunderbird applications.
For Firefox 21 Beta the  `about:addons-memory` measurements decreased from ~190k to ~130k.
For Firefox 22 the savings are about ~20k only due to [zone][1] landings.

[1]: https://blog.mozilla.org/nnethercote/2013/03/20/memshrink-progress-week-91-92/