Or better yet, use Tabs Outliner. Sidewise is dead. I have 8000+ tabs in TabsOutliner. No performance issues. Never lost a single tab (unlike the thousands lost with Sidewise).
Just use Tabs Outliner. It's pretty much nearly the same. I've switched to that about a year ago and haven't lost a SINGLE tab. It also makes backups that are REALLY easy to pick and restore which tabs you lost.
Customer support service by UserEcho