In Alfred 3.5, the Bookmarks feature was updated to include Safari and Chrome by default.
Take a look at Alfred's preferences under Features > Web Bookmarks to set up your preferred way of browsing these bookmarks.
Chrome bookmarks support
From Chrome users, please note that only the "Default" and "Profile 1" user profiles are being searched. If your Chrome bookmarks don't appear in Alfred's results, keep an eye out for Alfred 3.5.2, which will include improved support for Chrome profiles.
As the demand for Firefox bookmarks support has been very low, we recommend synchronising your bookmarks to Safari, and using the built-in Bookmarks feature.
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.