Can I change the format of numbers and date in the web client?

The values in numeric and date fields are always formatted depending of the language settings in your browser. The key is the first specified language in the list. The Web Client will be displayed in this language automatically. Numbers and dates will be formatted according to the language setting then. You can change the the lanuague as well as the formatting in the web client settings manually.

Generally these values are always saved in the database in a fixed format e.g.: "2016-05-31 ..." 

For example:
You open Web Client using a browser with English (USA) as first language in the list and change any date of a document to 05/31/2015. Now another user opens the same entry using a browser with English (United Kingdom) as first language in the list. He will see 31/5/2015.

This is only for numeric and date fields. If you save a number or date in any text field, then this value will be displayed in the way it was saved, nevermind which language is used!

Mozilla Firefox doesn't provide language settings. You need to download and install the desired language version. If you want to use e.g. English (United Kingdom) you need to download and install Firefox version "English (British)".