You can use redirects, but you might have to set them up on the web server level. Setting a front page redirect up in Drupal might be too late in the process. So:
Give the path prefix highest priority in the language negotiation settings. Else, URLs would not reliably select a language version. Already done in your case.
Find a URL that loads your French front page content. Options:
* Something like `http://example.com/fr/node/1`. For example, if you use Panelizer to panelize a node and to add views to it. Simplest option, but does not look good.
* Something like `http://example.com/fr/home`. Uses the node alias instead of the node ID.
* `http://example.com/fr`. Nicest URL. If it does not work for you, then possibly because you use Drupal's default front page config field, with which the language switcher does not work. You can make it work by enabling multilingual front page configuration [[see here][1], or [here][2] for more details).
Set up a redirect in the web server. For example if you use Apache and the "nicest URL" from above, you can use the mod_alias mechanism in
.htaccess:Redirect 301 / /fr
(If this does not work for your case, see some details.)
