That's no good. I'd like to troubleshoot that bug with you if you wouldn't mind. I've tried importing several old search.json.mozlz4 files and can't duplicate the error. If you have a github account, I'll open an issue there.

What platform, FF version are you on?

There's a 15s timeout for the import function while it tries to download icons. Does it hang after the timeout?

Are there any errors in the developer console when you try to import? (url bar -> about:debugging -> Enable Add-on debugging -> ContextSearch web-ext -> debug )