Even though this can work to fill a field (or retrieve all that text you just lost?), the interface is a mess of confusion and complication, even for engineers, and it cannot just simply fill a field as expected by clicking. I originally got this add-on because of losing hours of work, but functionality was so obfuscated this just kept happening to me anyway until I created saved documents for pasting to webpage fields. So if you need to fill multiple fields or forms EASILY, I think learn the Firefox built-in FormFill first.
Normally just simply clicking the toolbar button would fill the field or form or at least open dialog to basic functions like setting up your data such as a saved address. But no, that only opens a read-only window (no modify) showing a creepy amount of key-logging the add-on has done (like ALL of your recent texts or bank site entries unless you blacklist those). Then within that window you see the address you want filled (aha-the 2nd click?), but no that only frustratingly winks an icon on and off, or click fast enough and another read-only window displays the same untouchable data again..
So how to ACTUALLY fill in an address? First left-click on the field. Then right-click on the field to get context menu. Then scroll to the bottom to find the sub-menu. Then scroll down AGAIN past something called "herstel tekst veld" to pick one of two identically labelled "Fill Fields" items which should work if you have already typed your address earlier.
If you have time to earn a PHD, you could instead flip the manual for this thing and invest in a good flatclicker. You can discover all the scattered, secret, tiny interface elements that change usability depending on whether you left or right click. Like different places to access "Options" or "Preferences" (same thing/different names/different interface). There is a place to fix default key bindings (might type a weird symbol character or may not respond at all) you'll have to scroll the window since it's made too small. If you get access to the main dialog window, and not the small dialog window which is identical looking, (sometimes called "Manage History" sometimes called "Advanced Manage History") then you have a chance to search for what it was you wanted to click-and-fill and then you can right-click (not left-click which opens the grayed out read-only box) to get access to copying that info to the clipboard and then when you scroll to close the window (because the Close button is permanently below the window size) you can then paste that info into your form field. Oh wait, you can't do that because the whole window disappeared with the webpage and all the tabs when you clicked the Close box. Yeah. You made the mistake of having the Options/Preferences open in a tab in the same window as your webpage form. Good thing though, because closing that window revealed that you had 37 "Manage History" windows open behind it that somehow got opened in separate instances without getting closed. Now whether you dare click the Close button in the lower right or the Close button in the upper left for all those windows...you are on your own.

Very helpful. The only issue I have is the animations and fades and all that. I'd very much love if I could turn all of that off. It slows everything down significantly.

Does not work for mobile. Can see what's saved but cannot paste.

Very unreliable. Doesn't show the latest entries by default, has a messy interface and truncates whatever you've typed randomly. Borderline useless except for finding short messages.