One thing you can try is; disable sync until the 'net' is online.

I have 4 different profiles that I sync. One profile is in use as default while the others are only used once in a while.

When I start the non-default profiles, they sync after several minutes.

Also, you may not need to sync everything.

The problem isn't that Sync works. The problem is that it doesn't work without Internet connection and external servers even when devices can connect directly to each other. It's not only Firefox Sync's problem, it's about many other clouds. I think such services should use the most appropriate communication channel between devices. If they are far from each other, synchronization through the Internet is the only way, but if they are in the same local network or connected directly, using Internet for sending any data between them doesn't have sense and sometimes can even be impossible. It's rather a feature suggestion than a bug report, but I didn't know where to ask it.

Sync doesn't work between devices, the Sync server sits in the middle of all the devices.

It is possible to set up your own Sync server that wouldn't use the Mozilla Sync servers. https://mozilla-services.readthedocs.io/en/latest/howtos/run-sync-1.5.html

But it is either the Mozilla Sync server or your own server. There is no automatic switching mode that would use a different server location based upon whether an internet connection was sensed.

Plus a different Firefox Account would be needed for the times that you would want to connect to the Mozilla Sync server and other times when you wanted to do a local server Sync session. The user would need to Disconnect from one Firefox Account and Connect to another, which would need to be done when the user knows the next time that Firefox was going to be used a different Firefox Account might be needed. Sync is designed to automatically connect to the Sync server shortly after Firefox is launched.

The manual "toggle" for the user to be able to use Sync at will and have it turned off otherwise was removed when the Synced Tabs feature was added in the early Firefox 5# versions.

You can provide Feedback to Mozilla to propose new features here: https://qsurvey.mozilla.com/s3/FirefoxInput/

You would have to setup your own Sync server and use that locally if you do not want to use or can't use the online Mozilla servers.

• https://mozilla-services.readthedocs.io/en/latest/index.html

I have a simple idea how USB sync may work (i.e. between PC and smartphone). It wouldn't require any connection mode different than standard mass storage. There just should be a file in phone's memory, which address would be known for both PC and mobile versions of Mozilla. This file could be read and written by both of them, providing a synchronization channel. It doesn't have to contain all of user's data - it may be generated when user wants to synchronize, contain only a part of data (to save memory) which changed after it's reported to have been read by the opposite device and be deleted after connection ends (then mobile Mozilla's data would be stored as it is now).