This extension uses by default Firefox feature to highlight all occurrences of the selected text. When enabled, just select text with your mouse, and all occurrences will get automatically highlighted. Another click in the document will remove highlighting.
Using custom mode, you get the possibility to change the highlighting color and to display search markers beside the scrollbar. This mode might have variable results depending on Web pages.
HighlightAll can also display the number of occurences of the selected text.
Enabling/disabling can be done with a keyboard shortcut, or you can set it to be always active.
In case you find a bug or for any other information, please use this form to contact me.