Currently open tabs on one device, which could number in the hundreds, can't be opened quickly on a second device unless they are saved as a Bookmark group and then opened on another device.
First, it would be helpful if we could transfer open tabs from one device to another with ease, without bookmarking the open tabs (which isn't possible unless done before you leave the first device).
Second, it would be helpful if we could resume a full session that we have had on one device, make changes (I.E. close some tabs, open new tabs), and then switch to another device and be able to resume the exact same session on the new device, reflecting all newly opened or since closed tabs.
To do this, I would suggest a way to save a "persistent session" for lack of a better term. This session can be opened/closed on multiple PCs, laptops or tablets, with the latest changes always being reflected each time it's accessed from another device.
I would advise caution with any solution which would simply transfer still-open tabs from one device to another as while it could be useful, it may be chaotic when returning to the original device as there may be duplicates or tabs which were closed on the second device but remained open on the first device which the user returns to. This would add a level of clean-up required each time the user moves to a device which had been used once before.
In my use of Firefox, I tend to have a large number of tabs open at once, effectively one large persistent session which covers several topics, with tabs split between: shopping, research (incl. rabbit-hole-research), online courses, news, and schedules of immediate importance (e.g. flights, stores or swimming halls).
I recently found myself needing to transfer all tabs to a laptop from my PC. The only quick solution I found, was to bookmark all open tabs (hundreds of them) and open them on the laptop. Thankfully the laptop did not have related tabs open. However, I find it impossible to merge the two sessions again, as it will create scores of duplicate tabs, and I will find when I return to the PC that many tabs remain which I have in fact finished with and closed on the laptop. The only interaction between the now two separate sessions between the PC and laptop is the "send tab to device" feature. While this is a useful feature, unfortunately I find I will finish with a topic on the laptop and send one or two key tabs to my PC for action later, only to later think better of it and wish I hadn't sent them as I will now need to close them on the PC. Once I get to the PC, I may not instantly recall this and waste time looking at the dozen new tabs I sent over the course of a week.
"Sync'd open tabs" options don't allow me to open the scores of open tabs on my PC on the laptop. I found myself opening them one-by-one, which isn't reasonable.
Regarding use of persistent sessions on devices with less memory (e.g. mobile phones) there may need to be some limit to how many tabs actively load up at once. I've seen great Mobile phone Firefox performance improvement over the last year or two, so I have no doubt the team can overcome any such challenge.