It's a rendering problem.
Different browsers handle web pages and apps differently. It used to be a lot worse "back in the day" but web developers/designers are gravitating towards more uniform standards when they design their sites/pages.
That particular section "Calendar" may have some code that FF can't digest.
See if this tip will help
rais's tip. Everybody's different and I set mine to -2.0 you just have to experiment to see what suits you.
You may have to also adjust the browser's view settings if you use anything like
NoSquint.
Check the other responders at rais's tip and here for more insight:
Firefox 22 changed browser screen resolution?
More:
iCloud website has massive usability issues - is it compatible? | https://support.mozilla.org/
Just saw your recent response, my advice is to completely remove Java. You will be pleasantly surprised at how little you do not need it, besides it is a constant security risk.
If you do find you do need it for certain sites or gaming you can always re-install it. It could even be a reason for your rendering problem, and if you need it for the iCloud it may help to have a clean slate. Don't forget, it is okay to run javascript, it is entirely different from java.
How to remove Java
JavaRa is an effective way to deploy, update and remove the Java Runtime Environment (JRE). Its most significant feature is the JRE Removal tool; which forcibly deletes files, directories and registry keys associated with the JRE. This can assist in repairing or removing Java when other methods fail.
Source:
JavaRa - SingularLabs
For the IE10 problem, try:
Checking in Tools >Internet Options >Advanced Tab >Accelerated Graphics, if
"Use software rendering instead of GPU rendering" is checked, uncheck it.
Then scroll down to Security:
If "
Do not Save encrypted pages to disk" is checked, uncheck it.
Re-start IE10, see if that helps