Searching Firefox and Chrome Bookmarks
By default, Alfred searches your Safari bookmarks. If your browser of choice isn't Safari, there are a few ways to include bookmarks into your Alfred results.
Sync your bookmarks to Safari
Many users like using Xmarks, a third-party application, to sync their bookmarks between browsers. Doing this will allow Alfred to find your synced Safari bookmarks.
Use a workflow to search your bookmarks
With the Powerpack, you can use a workflow to search your Chrome and Firefox bookmarks. Take a look at Packal for the latest and best workflows, as these are updated to keep up with the frequent updates to the browsers themselves.