After installing the latest version of FF the buttons to scroll through tabs are missing.
Which buttons are you referring to?
Your issue can be caused by an extension that isn't working properly.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).
The little < and > buttons should appear at either end of the tab bar when you have so many tabs that they reach the minimum allowed width. Are those the ones that are missing?
Can you check your Tab Mix Plus options:
Display > Tab Bar > "When tabs don't fit width" drop-down