Version 5.7.4 is Available & Works with FF 13.0.1 Rated 5 out of 5 stars
I have used UnMHT for years, and love it - especially the ability to save just a selection of the webpage. The latest version offered on the Mozilla Add-Ons site as this is written (5.7.0) is not the latest version offered by the author. Version 5.7.4 is working perfectly on Firefox 13.0.1 (see below - the .RDF must be modified). The updated version (5.7.4) is available on the author's website (www.unmht.org). That site was down for at least a month, but has been redesigned and has been up for about two weeks, as this is written. I don't know why the author hasn't updated the Mozilla Add-Ons site with the new version link.
NOTE: UnMHT version 5.7.4 is set by the author to be compatible with Firefox versions 1.0 to 12.0. Here's the procedure to update ANY Add-On to install in newer versions of Firefox than the author's last offered update:
* Download the .XPI installer file (right-click the installer link, then left-click and "Save Link As", or "Save Target As"), and choose your desktop as the Save-To destination.
* If you have an unzip utility (7-Zip, WinZIP, WinRAR, etc) with shell integration, right-click the downloaded installer, then select your unzip utility, then click "Open Archive", "View Archive Contents", or whatever your unzip utility offers. (IF your unzip utility doesn't offer right-click shell integration, open the utility, use it to navigate to the add-on installer .XPI, then open the .XPI with the unzip utility. (I use the excellent freeware, open-source, "7-Zip"; and installed it with shell integration).
* Once you can see the files inside the .XPI installer, double-click the "INSTALL.RDF" file, to open it in your text editor, such as Notepad (you might have to associate the .RDF file extension with Notepad, before you can open it).
* Do a text search in the .RDF with your text editor for "max" (without the quotes), to find the section which sets the Minimum and Maximum Firefox versions that the .XPI can be installed in. [In Notepad's menu bar > click "Edit" > click "Find" > type in "max" (without the quotes) > click "Find Next"]. MAKE SURE YOU ARE IN THE FIREFOX SECTION - the UnMHT add-on has separate min-max sections (<em:targetApplication>) for different browsers (Firefox, Prism, Fennec, Thunderbird, SeaMonkey, Flock, Netscape).
* Change the "maxVersion" string (example: <em:maxVersion>12.*</em:maxVersion>) to reflect a newer
upper range (example, with a little future-proofing: <em:maxVersion>15.*</em:maxVersion>). [This requires changing only one character in the file: "2" to "5", in the above example].
* In the 'future-proofing' example above, I have set the add-on to install into any version of Firefox up-to-and-including all versions (and subversions) of Firefox 15.
* Click the text editor menu > "File" > "Save" to effect your modification. The unzip utility may popup a dialog (7-Zip's dialog states: "File 'install.rdf' was modified. Do you want to update it in the archive?"). Click "OK" or "Yes" to complete the modification. Then exit the unzip utility.
* If you ever update an add-on this way and it DOESN'T work with your new version of Firefox, simply "Disable" the add-on in Add-Ons Manager; then wait for a compatible update to be offered by the author or other users.