I wouldn't take the incompatibility warning too seriously... Firefox can become sluggish in various circumstances. The endless scrolling design of Facebook causes Firefox to consume a lot of memory. As Firefox's memory consumption reaches 1.5 to 2.5 GB, it tends to bog down. Often the only remedy is to exit out of Firefox and start it up again (just closing a couple tabs usually doesn't significantly reduce memory consumption). Some users with slower hard drives also find that there is a lot of disk thrashing as Firefox uses the disk cache. If your drive is very fragmented or cached access seems slow, you can clear the cache to see whether that helps, and you can reduce the size of the cache -- which works best with a fast connection. For clearing Firefox's cache, see: [[How to clear the Firefox cache]] . If you have a large hard drive, this might take a few minutes. If you do not see the number going down on the page, you can reload it using Ctrl+r to check progress. To reduce the cache size, use the checkbox below the Clear Now button to override the computed optimal size, and choose something more modest. Does that help?