I was prompted to upgrade mainly because Thunderbird constantly keeps asking me whether I want to upgrade it, but it never works if I just click on the link. I normally wait until it gets upgraded in the next distribution.
However, I thought I would investigate Firefox18 just to see if a manual installation would work before I tried Thunderbird. My email is more important to me than a browser!
So I followed the on-line instructions which worked fine, BUT if you do the same as I did, watch out for the bit where they tell you to remove the old installation!! I did, which is OK unless you close the browser you're working from, ( which I did by mistake) and then hey presto, you have no browser, no on-line guide, no nothing! Fortunately I used my Android to quickly browse to the same page and I followed the rest of the instructions and everything is now fine!
Just thought it might be a useful cautionary tale for those users who like myself are not that familiar with new installations etc.