Hi, to find and highlight strings on the page you need to click the 'Find' button.
I use this extension myself almost every day and it definitely works in such a simple case.
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Hi, again.
I tried to check what could be wrong between firefox56 and my extension. And it seems firefox56 just doesn't support text searching on a page at least in a form as current browsers do.
As I can see 56 version was issued 1.5 years ago, and some things must have changed in such a timespan.