Beagle is (was?) a concurrent desktop indexer, built on Lucene and the Mono project (C#), for which a number of add-on browser plugins were written. These work by copying visited web pages to an indexing queue directory, which the indexer then processes. Especially, there is a Firefox extension.
If, for any reason, you so happen to prefer Recoll to Beagle, you can still use the Firefox plugin, which is written in Javascript and completely independant of C#, Beagle, Lucene..., and set Recoll to process the Beagle queue directory. This supposes that Beagle is not running, else both programs will fight for the same files.
This feature can be enabled in the GUI indexing configuration panel, or by editing the configuration file (set processbeaglequeue to 1).
There are more recent instructions about how to find and install the Firefox extension on the Recoll wiki.