X-notifier 3.3.3 and later
Topic refers to:
extensions.xnotifier.accounts.[HOST#YOUR_ID].link [and set it the URL you want.]
1) Type 'about:config' in the location bar.
2) Look for HOST account with 'enabled' [Right-click] Copy Name
3) [Right-click] New => String
4) [Right-click] => [Paste] and replace 'enabled' with 'link'
5) [Click] OK
6) Enter string value (see below)