Add support for Firefox 1.5+ & Co
Notes:
- SeaMonkey older than 2.0 uses very old extension manager (and not supported)
- we had to use Components.utils["import"]() for Firefox 1.5
Fix typo: correctly report error for wrong exclude pattern
Legacy loader: load bootstrap.js after "profile-after-change"
(we may receive "nsPref:changed" for each changed preference otherwise)
Also checking for second call looks useless in this case
Update changelog: support for Firefox 1.5+ & Co