Firefox doesn't sync search engines. Yahoo! is the default for new installs of Firefox.

In some cases, there are preferences for whether to sync preferences. You could take a look at what is set here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste prefs.sync and pause while the list is filtered

All of the ones I see are set to true, but I do not see a sync preference corresponding to the preference that stores the selection and order of buttons on the toolbars, "browser.uiCustomization.state". Perhaps that's because the service was originally designed for exchanging data with mobile versions that do not have the same concept of a main toolbar?

soundblaster16 said

However every time I make any layout, menu, toolbar changes, those won't sync. And not all preferences sync either. ---- For example it keeps defaulting to Yahoo. What am I missing? This is for new installs of Firefox on one of my machines or current installs.

Not all preferences will Sync, especially when it comes to add-ons - including those that have toolbar buttons. But most of the other UI (user interface) prefs should synchronize. The "problem" is prefs that are hidden in about:config that Mozilla hasn't documented too well, if at all, in a centralized location here in the Support KB.

As far as the default Search engine setting, I suspect that might not 'sync'. But I can't say for sure - I quit using Sync on a regular basis before Firefox 34 came out with massive changes made to the Search layout and preferences. I just have no use for multiple devices since I retired a few years ago.

One thing that can throw off Sync is any differences in extensions between separate desktop / laptop Firefox installations. And as Jefferson mentioned, "Mobile" (Firefox for Android and for iOS) was a prime consideration when the Sync service was expanded 2 1/2 years ago to include "add-ons" - which doesn't seem to include Search Engine type "add-ons".