Apt-get saying "Unable to correct problems, you have held broken packages."

Possible Duplicate:
How do I resolve unmet dependencies?

TL;DR: sudo apt-get install ... saying "Unable to correct problems, you have held broken packages."

I was trying to get the WebApps feature for PP and QQ following this blog post.

I ran the sudo add-apt-repository ppa:webapps/preview command to add the repository, but i got a connection error. Since I know my current ISP gives a shaky connection, I tried again and sure enough, it worked.

Then I ran sudo apt-get install unity-webapps-preview, but I realized we had to update apt-get first, so I hit Ctrl + C to stop it. Then I ran sudo apt-get update which worked w/o a fuss, but when I ran sudo apt-get install unity-webapps-preview again later, it showed an error message. Here's the dump:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 unity-webapps-preview : Depends: xul-ext-unity but it is not going to be installed
                         Depends: xul-ext-websites-integration but it is not going to be installed
                         Depends: xul-ext-webaccounts but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

I think this might be because of me interrupting the earlier command. It hadn't got a chance to output anything, though — I stopped it pretty fast.

Commands

I tried these:-

• sudo apt-get install --fix-broken


• sudo apt-get autoclean


• sudo apt-get autoremove


• sudo apt-get -f install

But even after running sudo apt-get upgrade after every try, none of them worked.

Purging

I tried purging the repository, but that didn't work either. First, I got the package using sudoapt-get install ppa-purgeand then tried to purge the WebApps repo usingsudo ppa-purge ppa:webapps/preview`.

Manually installing dependencies

If you look at the error dump carefully, it says:

Depends: xul-ext-unity but it is not going to be installed
Depends: xul-ext-websites-integration but it is not going to be installed
Depends: xul-ext-webaccounts but it is not going to be installed

So I tried installing them manually. I ran sudo apt-get install xul-ext-unity, but got this:

Depends: xul-ext-websites-integration but it is not going to be installed
Breaks: firefox (>= 15.+) but 16.0~b6+build1-0ubuntu0.12.04.1~mfn4 is to be installed

I have no idea what the second part means, but I tried resolving the first. Running sudo apt-get install xul-ext-websites-integration gave:

xul-ext-websites-integration : Breaks: firefox (>= 15.+) but 16.0~b6+build1-0ubuntu0.12.04.1~mfn4 is to be installed

As a last resort, I tried sudo apt-get install xul-ext-webaccounts, but that gave the sane message too:

Breaks: firefox (>= 14.+) but 16.0~b6+build1-0ubuntu0.12.04.1~mfn4 is to be installed

What does Breaks: firefox (>= 14.+) but 16.0~b6+build1-0ubuntu0.12.04.1~mfn4 is to be installed mean?

I tried searching Google, looking at a couple of forums, looking at this question and searching on AU, but to no avail. Help would be appreciated.