By itself it doesn’t look like a big change, but it is the accumulation of such good work that will make Firefox snappier.
Some more insights:
Firefox uses a cache when transforming a text into a series of glyphs to display on screen. The transformation depends of the text itself, but also to the CSS rules applied to it (selected font, bold, italic, ligatures, …). This is a fairly complicate stuff and therefore takes time.
The cache memorizes previously transformed text and allows Firefox to skip previously done work by retrieving it out of the cache.
That’s this cache that has been replaced by a more efficient one.
The CSS3 Multi-columns layout has reached Candidate Release in April 2011. Since years, Firefox implements most of it, prefixed with -moz.
But there are some differences, as the spec slightly evolved since it was implemented in Gecko. In order to be able to unprefix the CSS3 multi-columns properties, there is an ongoing action to adapt Gecko’s implementation.
Now, like a late Christmas gift, it is the column-fill CSS property that is coming (prefixed) in your next Nightly. Column-fill lets you define how the different columns are filled: either the algorithm used will try to balance their content, trying to have them filled with more or less the same amount of data, or the algorithm will just fill them sequentially.
That’s really nice to see this in Gecko.
There are still a few missing features, like the column-span CSS3 property and the three ones related to column breaks: break-after, break-before and break-inside. Once these will be there, it will be time to unprefix all the CSS3 Multicol related stuff.
These CSS properties are implemented in this very same Nightly.
Note that border-image also support the fill value now.
Finally, it is planned to remove the prefix very soon (probably the next month) as the specification is now stable and reached the Candidate Recommandation status, and the implementation in Gecko matches it now.