There can be some packages that depend on (or recommend) a browser, in the form of a virtual package (firefox provides the virtual packages www-browser, iceweasel and gnome-www-browser) or through an alternative (some package depends on firefox | chromium-browser).

To solve your problem you can use aptitude that provides more choices to as to proceed. The package aptitude is not installed by default, so you need to install it

sudo apt-get install aptitude

then you can do

aptitude --simulate remove firefox

and see what it suggests. This is what it says on my system

enzotib@laptop:$ aptitude --simulate remove firefox

The following packages will be REMOVED:  

  firefox 

0 packages upgraded, 0 newly installed, 1 to remove and 19 not upgraded.

Need to get 0 B of archives. After unpacking 30.6 MB will be freed.

The following packages have unmet dependencies:

  icedtea-plugin: Depends: firefox (>= 4.0~b7) but it is not going to be installed. or

                           chromium-browser but it is not going to be installed. or

                           epiphany-browser but it is not going to be installed. or

                           midori but it is not going to be installed.

  firefox-globalmenu: Depends: firefox (= 5.0+build1+nobinonly-0ubuntu0.11.04.2) but it is not going to be installed.

  firefox-gnome-support: Depends: firefox but it is not going to be installed.

The following actions will resolve these dependencies:



      Remove the following packages:                                                             

1)      firefox-globalmenu                                                                       

2)      firefox-gnome-support                                                                    

3)      icedtea-plugin                                                                           

4)      icedtea6-plugin                                                                          



      Leave the following dependencies unresolved:                                               

5)      totem-mozilla recommends epiphany-browser | www-browser                                  

6)      ubuntu-desktop recommends firefox                                                        

7)      ubuntu-desktop recommends firefox-gnome-support                                          

8)      moonlight-plugin-mozilla recommends firefox | abrowser | iceweasel | xulrunner-1.9.2     

9)      ubuntu-restricted-addons recommends icedtea6-plugin                                      

10)     xul-ext-ubufox recommends firefox (>= 4.0~b6) | abrowser (>= 4.0~b6)                     

11)     mozilla-libreoffice recommends iceweasel | firefox | iceape-browser | konqueror-nsplugins





Accept this solution? [Y/n/q/?] 

This way you can see removing firefox what packages breaks. If you choose n, another possible solution is suggested, ans so on.

When you have a best understanding of the problem, and you have selected your way, run aptitude without the --simulate option.