Ok. This extension used to be *bad*. It was limited in its use and had a serious problem with what I'm terming "archive misfire", where it would decide to redirect some pages (which would otherwise load just fine!) to a new page where it gave you the option to view it in the archive. When you didn't need to.
Yet now! As of February this year, the extension has leapt all the way from version 1.8.6 to version 3.0, and let me tell you, this version absolutely slaps. It's brilliant. Archive misfire is no more, and it's got all sorts of helpful little options. The Internet Archive has taken this extension from an over-agressive bookmarklet to a really quite helpful browsing tool. If you scorned this extension before, I invite you to give it another go; and if you're here for the first time, don't be scared off by the review score - truly, things are better now. Also, some other reviews mention needing to login to save pages, a requirement which has been lifted as of ~a month ago.

This is a great extension. It is much quicker to save a page than it used to be, and has quick links to archived versions of pages. The auto-archive feature is brilliant.

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.