Wrong Favicons in Firefox
Close firefox, delete %AppData%\Mozilla\Firefox\Profiles\<your profile>\favicons.sqlite
This is easier to do than peqhusus's solution, but it does mean you'll lose all cached favicons and have to reload your bookmarks to get the icons again.
This may not be the best solution if you are only slightly annoyed...
What I usually do when this happens is go to
Bookmarks --> Show All Bookmarks --> Import and Backup --> Export Bookmarks to HTML file
Then open the bookmarks file, find the corresponding bookmark entry, then change
ICON_URI="{URL}" ICON="data:image/png;base64,{data}
to the URL of the icon and the base 64 encoded icon in png format
Then import the modified bookmarks file back into Firefox