The foundation Mozilla decided to add a new style to the Firefox preferences UI. It is already used by default in Firefox 36 beta, but you can enable the new UI manually in the current stable version of Firefox 35 as well.
Let's see how you can open the new preferences page and what's different.
To enable the new preferences in Firefox 35, type the following address:
about: preferences
Press Enter and wait a few seconds (the first time will be delayed). It will open a window like the one you see in the picture below:
Unlike the dialog in the old ones settings, these preferences appear on the right inside a tab. The settings are not organized into tabs by category, but are categorized by lines on the left.
The design reminds the Firefox add-ons manager. Apparently the Mozilla Foundation displays settings in this way to give a consistent look to all operating systems and devices.
If you also use the Google Chrome you will notice that its settings are similar to Mozilla's new UI. This shows that the Mozilla foundation continues to copy Google Chrome's design. The new settings are slower than the old ones, however, they have some advantages. For example, the search settings seem to be more comprehensive compared to the old ones.
It is noteworthy that its Nightly version Firefox has these preferences enabled by default.