I have a similar problems... on FreeBSD 10.1 STABLE with Firefox and pentadactyl both built from source.
Firefox-esr may still work; but I haven't tried yet.
I do have keybindings but no tab completion at all. Some of the error messages I get:
/home/x/.pentadactylrc:13: E518: Unknown option: altwildmode
/home/x/.pentadactylrc:14: E518: Unknown option: autocomplete
/home/x/.pentadactylrc:15: E518: Unknown option: wildmode
/home/x/.pentadactylrc:16: E518: Unknown option: complete
Invalid argument: defsearch=s: CompletionContext is not a function
Invalid argument: suggestengines=: CompletionContext is not a function
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined
resource://dactyl-content/commandline.js: 1107: ReferenceError: CompletionContext is not defined