Like many of you, I tend to operate with a huge number of open tabs...impossible to manage except in tree mode. I migrated from Firefox to Opera because TreeTabs worked more reliably, but gave up after 2 years full of crashes, lost data, & the author's failure to either maintain the plug-in or respond to user questions & needs.
Kudos to the TreeTabs author for great design, but I vowed never again to rely on a free plug-in for such fundamentally essential functionality.
I moved to Vivaldi nearly a year ago. It's quirky, & I choose to ignore some key features...but a variant of tree-style tabs is built-in & bulletproof. The whole venture is run by Opera veterans, & it shows. I'm satisfied overall, & suggest Vivaldi is worth a serious look.