Updated after user answer:

Thanks a lot for the detailed explanation of the issue. I promise that I will go through each one and investigate it thoroughly to find the bug.

Regarding the other 2 issues:
1. The scrollbar is awfully small on purpose, let's just say by design (I thought that a non intrusive scroll bar is better because I expected most people to just use the mouse scroll )... of course, it doesn't have to be like that, it can be stupid small and I totally understand that. I will make sure to add an option to configure it to be just normal. Regarding the interference from the popup with the scrollbar... this one also on purpose because it looked a little out of place to move it near the scrollbar. Not really sure who to approach some things having in mind that there's no real solution to place everything on screen and not affect something else. I will think about it. The fact that the mouse can be glitchy sometimes... I'm afraid that can't be something from be but most probably from the addon popup window (if in the Tab View it doesn't happen then that's for sure)

2. Need to investigate that further, because it seems really weird

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Are you sure they are not just in reverse order ? Depending on certain Firefox setting each user has, tabs can be restored in reverse order or in the normal order (If for example you activated new tabs to be opened next to the current one or similar settings). This is not something I can control myself since each user can affect the order of the tabs. I just simply tell firefox to open all the links no matter how many they are. If there are 1000 tabs I tell Firefox to open all of them in the right order they were saved. I wish there was something wrong with the way the Addon was built but trust me there's really no issue. There can be multiple settings (from other addons or just Firefox settings) that can mess up what I tell the browser to do.
That's why I added the "Reverse restore order" feature in the Options. Please check that one to see if order will be preserved.
The addon knows to open even more than 10000 tabs, but depending on the number you want to open I'm not really sure if Firefox can really handle it (more exactly if the PC hardware can handle it the correct way). The thing is, if there's really a bug in the way Firefox opens 100 tabs when I tell him to do that I can't really do anything about that.

Anyway, I will investigate this again, but there's simply nothing wrong with the addon, I do what I must (I mean.. really... I go through every single tab and request Firefox to open it... it's not like I can't really do it properly), but if tabs are not opened then I probably have to check if there's any limitation regarding what the addon API can do, or if Firefox might miss some messages from the addon and I will try to find some workarounds.

Regarding your request to show the number of tabs next to each session. Yes, sorry about that, I will try to land it in the next update (the feature is planned but I kind forgot about it).

Regarding the rating, please have in consideration that there are extremely few people that really have hundred of tabs opened and restored so even you if are totally right the review might not be in line with most of the users. I really try to improve it constantly and even if I do something wrong I try to fix it right away. Having support from the community it's what drives me to continue to improve it :). Issues are fixable, but sadly bad ratings remain no matter what I do :(. No, I'm not asking to change your review, I just wanted to point out that it may not be a very accurate review for whoever intends to use the addon in the future.