Firefox ACCOUNTS only store so much history, the main reason why I created this extension. If you mean that not the entire history is imported properly it's either just slow at updating or there's somehow a bug still. As far as I know it should just import everything. It did last time I used it.

Edit: Sigh, looks like there's an issue still with importing a large amount. I can't import my own history (filesize ~110mb) anymore without the browser locking up. I'll have to have a look at this some day. The JSON isn't that intricate, it shouldn't be hard to manually cut it up into smaller chunks for now.

Edit 2: Adding a very noticeable delay in importing (my own importing takes ~9 minutes with this test) I'm importing everything fine. I was getting complete freezes and even an out of memory hard crash with the available version, so I guess that's an improvement but I'm not happy with it being so slow. It would also be hardware dependent what delay would be appropriate and I'm not really willing to figure out a general way to set a hardware specific delay.