[R-SIG-Mac] Cannot install packages after update

[Christiaan Pauw]

Hallo Everybody

I fear I may have broken something. I tried to use firefox as my help
browser in Rstudio becuase the help window is really small. I tried
options(browser = "/Applications/Firefox.app/Contents/MacOS/firefox")
but that didn't work. Thereafter I couldn't use my help window in R
studio from here on. I updated R and R studio. When I click on the
Packages & Data menu item in the (normal) R Gui I get the following:
Error in readRDS(file) :
  ReadItem: unknown type 32, perhaps written by later version of R

If I try to use the package installer and check for packages I get:
Warning: unable to access index for repository
http://cran.za.r-project.org/bin/macosx/leopard/contrib/2.15

I cannot do ity from the consule either:
> install.packages("plyr", lib="/Users/christiaanpauw/Library/R/2.15/library")
Warning: unable to access index for repository
http://cran.za.r-project.org/bin/macosx/leopard/contrib/2.15

I am almost certain the problem started when I broke the browser
option: I couldnot find the correct option for Mac anywhere
it is currently set to
> options("browser")
$browser
[1] "/usr/bin/open"
in RGui and :
> options("browser")
$browser
function (url)
{
    .Call("rs_browseURL", url)
}
<environment: 0x100a0c028>
in Rstudio

Does anyone have an idea how to fix this?

Thanks in advance
Christiaan

P.S Here is my sessionInfo  and libPaths:

> sessionInfo()
R version 2.15.3 (2013-03-01)
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] tools_2.15.3
> .libPaths()
[1] "/Users/christiaanpauw/Library/R/2.15/library"
[2] "/Library/Frameworks/R.framework/Versions/2.15/Resources/library"