Firefox browser’s Find bar, which lets you search for words and phrases on a page, to get three changes: the find toolbar includes an option to search for whole words only, and it highlights every instance of the search phrase you’ve entered by default, instead of you without the need to click on Highlight All button, and it also dims the page as well.
Mozilla internally calls better highlighting and page dimming as modal highlighting feature, you can observe this behavior of Find bar for page searches in Safari and Opera 12.x browsers also. If you want to disable the new changes added to Find bar, you can, follow the steps mentioned below for that.
Disabling Firefox Find bar’s page dimming and highlight all features
I. To turn off Page dimming, visit about:config and change the preference findbar.modalHighlight value to false.
II. Similarly, if you don’t want Firefox to highlight all occurrences of a search term in a page, change the preference findbar.highlightAll value to false.