I'm currently updating the add-on to 3.5, improving the user interface and maybe add some new feature. But for now if you wish to use the extension with Firefox 3.5 you can use a little hack that works with no problems.
Just locate your Firefox profile folder, and then open the
extensions folder inside it. There will be one folder with the word "hashr" in the name. Open it an edit the file
Inside the file locate the following lines:
And change it to:
Save the file and restart Firefox. It should now be working perfectly. If not, check the add-ons manager to see if it's disabled.
Edit: Mozilla has finally approved the new version of hashr, so this hack is no longer needed. You can find the addon here: https://addons.mozilla.org/en-US/firefox/addon/8539.