The first diagnostic step is to run in safe/troubleshoot mode (hold Shift when launching TB). If that doesn't help, run Windows in safe mode and then try TB in normal mode. If that shows improvement, you probably have to disable MWB from scanning the TB profile (add an exclusion for the profile folder), leaving the real-time background scan on.

Thanks kindly for your help! When I run TB in safe mode as instructed, it behaved properly, only soaking up ~1 GB of RAM (and I have several enormous inboxes, so I *think* that makes sense?).

I wasn't sure if your advice to add TB's profile folder as an exclusion to MWB "if that shows improvement" was meant in the <TB in safe mode> scenario, or the <win 10 in safe mode> scenario, or either; is that what I should do? Since I disabled all of my extensions (a list of which I don't know how to find), is that part of the problem? Should I disable all and add one by one until the problem reappears? Now that I think of it, the timing may have coincided with me adding the UK language pack ...

Lastly, I would like to support the right organisation for your excellent and swift support, but I'm not sure which. I'm obviously going through Mozilla support, but I thought that TB was no longer Mozilla ... any clarity on this would help me with a donation. :-)

Language packs are a known issue with TB 91, so that alone might be the major factor affecting performance. But it's still worth testing if any others, under Tools/Add-ons & Themes, Extensions, are a factor.

Adding the profile folder as an exclusion in MWB is also worth testing with TB in safe mode (which disables the language pack); MWB is disabled entirely in Windows safe mode. Here's what I suggest: run Windows in safe mode and TB in safe mode, then compare performance with Windows in normal mode and TB in safe mode. This isolates the effect of MWB, if any, since you've already seen that TB in safe mode performs better than TB normal mode.

If MWB has a negative effect, you should definitely exclude the profile folder from MWB. If MWB has no obvious effect, you can keep its settings as before (although excluding the profile is recommended for all such AV apps).

The volunteers on this support forum are not connected in any formal or financial way with TB. Donations support the development of the software and associated infrastructure.

Thanks again for your advice. I tried several combinations according to your suggestions, and here are the results:

Windows in normal mode:

• TB safe: no memory creep, starts at ~0.7 GB, hits ~1.2 GB RAM
• TB restart w/ add-ons disabled: memory creep, TB won't load
• TB restart w/ add-ons disabled & toolbars/ctrls reset: memory creep, TB won't load

Windows in safe mode:

• TB normal, Win safe: memory creep, TB won't load
• TB safe, Win safe: no memory creep, hits ~1.3 GB, plateaus

This suggests that MWB has no effect (is not part of the problem). Since using the "TB restart w/ add-ons disabled & toolbars/ctrls reset" feature doesn't solve the problem, does this mean I have to use TB in safe mode all the time, now?

Thanks for clarifying the relationships between volunteers and Mozilla; your support is appreciated. :-)

TB safe mode does other things besides disabling add-ons and using the default layout. The next most likely factor is hardware acceleration (disabled in safe mode). If it's enabled in TB normal mode, (under Preferences/General/Indexing) try unchecking the box, restart TB in normal mode, and see if there's a difference.

Your solution worked, though I got curious and tried a few things before I got there, which also worked, strangely. First I noticed that there was a new update, 91.2.0, but that update didn't work any better. So I thought I'd try an older update from August when all was working fine; I tried 91.0, and it worked fine in normal mode. I next unchecked the HW acceleration box as you advised, and updated to the current 91.2, and that also now works. So something went sideways between 91.0 and 91.2, I believe, at least as regards HW acceleration.

I thought I'd give you the gory details in case it a) helps you, b) helps you help somebody else, or c) allows you to tell somebody at TB/Mozilla who can use that to troubleshoot something?

Anyways, your support is *much* appreciated. I like Mozilla, TB, and want to keep using it. I have tried other mail clients, but for me, nothing really compares to TB's layout, search speed, flexibility, information density, ... I really like it.