Quick solution
Install Vimperator 3.7.1 instead of Vimperator 3.8. Go to https://addons.mozilla.org/en-US/firefox/addon/vimperator/contribute/roadblock/?src=version-history&version=3.7.1 and click 'Add to Firefox' button.
Long solution
This is same as the previous solution. The steps below document how you can reach the URL mentioned in the previous solution from the Vimperator addon page in a step by step manner.
- Go to https://addons.mozilla.org/en-US/firefox/addon/vimperator/
- Scroll down to 'Version information' section. Click on this section.
- At the bottom of this section, click on 'See complete version history' link. You'll land on https://addons.mozilla.org/en-US/firefox/addon/vimperator/versions/.
- Scroll down to 'Version 3.7.1' section and click on 'Continue to download'. You'll land on https://addons.mozilla.org/en-US/firefox/addon/vimperator/contribute/roadblock/?src=version-history&version=3.7.1.
- Click 'Add to Firefox' button.
Why does Version 3.7.1 work but Version 3.8 doesn't?
Visit https://addons.mozilla.org/en-US/firefox/addon/vimperator/versions/. Scroll down to Version 3.8, then click "View source" link below it. The install.rdf file of Vimperator 3.8 shows that Vimperator 3.8 can be installed on Iceweasel/Firefox versions 20.0-26.0.
<em:targetApplication> <Description> <em:id></em:id> <em:minVersion>20.0</em:minVersion> <em:maxVersion>26.0</em:maxVersion> </Description> </em:targetApplication>
Now visit https://addons.mozilla.org/en-US/firefox/addon/vimperator/versions/ and scroll down to Version 3.7.1, then click "View source" link below it. The install.rdf file of Vimperator 3.7.1 shows that it can be installed on Iceweasel/Firefox versions 17.0-23.0.
<em:targetApplication> <Description> <em:id></em:id> <em:minVersion>17.0</em:minVersion> <em:maxVersion>23.0</em:maxVersion> </Description> </em:targetApplication>
Therefore, install Vimperator 3.7.1 from https://addons.mozilla.org/en-US/firefox/addon/vimperator/contribute/roadblock/?src=version-history&version=3.7.1.