Scanned through my ~1000 add-ons in like a couple of minutes. Helped me get rid of a dozen dead ones, and fix the redirection of ~150 ones.

A couple of suggestions:
1) Some 'select all', 'select multiple', 'select none' and also checkboxes for each bookmark entry to offer greater control over which bookmarks get changed. For example, out of the 155 re-directions the add-on wanted to fix, I wanted to leave ~10 as it, while let the add-on correct the rest. Clicking 'correct redirect' for each entry would have been tedious, and clicking the 'correct all redirects' wasn't what I wanted. I eventually found out that clicking 'edit' then 'Save' for a bookmark marked as a warning excludes it from the 'correct all redirects' action, and this saved me a lot of time and effort.
2) If the webextensions API allows for this, offer to create backups, or otherwise encourage the user to create them on their own before they use the add-on to make changes. This could help users from shooting themselves in the foot.
3) Every time I clicked edit for a bookmark and then clicked Save, the Keefox popup showed up and tried to 'save the login.' Could this be fixed on your end or is it an issue with Keefox?
4) Once the results of a scan for 'missing bookmark names' show up, scroll down a bit so that the header changed from class `default` to `compact`. This compact mode doesn't feel compact enough. For example, a resolution of 1366x768 gives me a content area of ~630px on Firefox. But even with the header in `.compact` mode, I'm left with the results displaying within a height of ~400px. So (230/630)*100=36% of the content area is being spent in showing the header / footer and that action bar with the drop down and the 'check bookmarks' button. Maybe make the action bar and footer a little smaller, and maybe have (at least the compact mode of) the header change the list ' Number of bookmarks', 'Checked', 'Errors' etc. into a horizontal one instead of a vertical one to save vertical space. There is already a lot of empty space in the compact mode of the header between the star on the left and this list on the right.

Two nitpicks :)
1) The search bar that shows up upon completing a scan says 'Name or URL...' It is not immediately obvious that the text typed into this would filter the results shown below. Maybe something like 'Type name or address to filter results' is better?
2) The fox image on the right in the add-on page is very cute, but the white fur of the fox is too similar to the page background. I couldn't tell what the image was until I right click > View Image'd it. Maybe place it in a circular background for better contrast? This would also help give it the appearance of a button and better convey to users that it is click-able.