Version 3.0 works fine here!
With the new feature, which shows saved status of the page right away, now it is very easy to make a decision whether or not to save the page. To do this with prior versions, I had to first check the newest saved page and to see if it exists or not. The saving process is being done in background, so I can browse other pages while the add-on saves a page.

(Below is my review a year ago about previous version)
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Once worked fine, but for the past few months, it has got "Fail with status: 498 No Reason Phrase" error with Retry button, which fails all the time on firefox (waterfox as well). So, now this extension is quite useless and needs to be updated.

Finally, they fixed this and it works perfectly.

The new v3 is very good

Love the app! Really helps me view pages that aren't available anymore. One problem though: Using the app with multiple windows will open tabs in the first opened window, not the current one.

Useful to automatically archive pages, and for finding archived versions of pages.

the new autosave feature rocks.

Really good, if site is not available but is saved, a pop-up appears asking if you want the saved version with a button to the link.