St. Clair Software updated their excellent Default Folder X utility to version 4.1.1 today, including the following bug fix:
Partially worked around an AppleScript bug in Firefox. This bug prevents other applications from getting the URL of the current page from Firefox when Default Folder X is running, meaning that applications like LittleSnapper and EagleFiler cannot take snapshots of web pages. If you still encounter this problem after updating to Default Folder X 4.1.1, try quitting and relaunching Firefox or add Firefox to Default Folder X’s exception list in the Advanced tab of its preferences.
This may resolve some of the problems other users have had in capturing web pages from Firefox using EagleFiler.