I've using Firefox on my Lenovo G590 Laptop for a long time. Now, I've migrated to Acer ES1-711G Laptop, which is much more powefull, and firefox is running much slower on it. By "slower" I mean: on Acer, when just 5-7 tabs are open, after click on tab, there are a second pause before switching to it. Interface elements, such us buttons or textboxes doesn't react on a click for the first time with 50% probability. Sometimes cursor in a text fields hangs for a half-second, when continues to move or type. Sometime, when I enter URL into address bar, and press ENTER, it doesn't react, and I have to do it again. This annoys me the most, because, I'm used to, that if some program working slow, it must, at least, do what I tell it to do, albeit slowly, but address bar just ignoring me! Normal UI lags, then I can click somewhere, type something, press ENTER, and sit back, waiting while program do all this things (like in Windows 95 on 386PC with 1MB of RAB). But this UI lags of firefox, that doesn't do what I tell it, because ignoring half of my actions - is unacceptible. At lenovo, firefox is starting to work slowly from time to time, when I opening around 40 tabs, but even then, it's just slowing for some time, and when returns back to a normal speed. It's never ignores what I'm telling it to do, albeit makes it no so fast. On Acer this condition is almost permanent. Comparision: Lenovo G590: 2-Core CPU at 1450 MHz, 8GB Ram, Debian 8 Jessie Acer ES1-711: 4-Core CPU at 2660 MHz, 8GB Ram, Debian 8 Jessie I've installed Debian on an Acer, and move all my home data into it, so, almost no configuration changes were made. I have all the extensions, that exists on a Lenovo installation (a lot of them!), but, on Lenovo they aren't make this horrible results. I've tried to reset my firefox account on Acer, and then reinstall all the extension. For the first time, after reset, firefox worked faster, but, after some time passed, it returned to a mentioned slowly operations, so I returned to my Lenovo's profile (copied it again from Lenovo). Extensions I have: flashblock, leechblock, SpeedDial FVD, TabMixPlus, Session manager, Always Ask, Custom Buttons, FireBug, FireShot, FirePath, friGate, FoxyProxy, GreasyMonkey with some scripts, Hide caption titlebar plus, IdentFavicon, Lazarus:Form recovery, OmniBar, SaveFromNet, HTTPS Everywhere. You might say: "Of course your firefox installation is slow! Look at all this extensions!", but on slower PC it works faster with all the same extensions, and exactly save profile directory. On Acer: All the time I work with firefox and running system monitor, I see, that i'ts eating up to 110% of CPU, and that's ok for me, because it's just one CPU Core, and I have 4! But firefox is single-process app :( I've tried to activate electrolisys multiprocessing feature, but most of my extensions is incompatible with it, so I can't use it now. Now, when I writing this text, I have 80-101% of CPU usage by FF. When I'm stopping to write, or do anything, even move my mouse, FF CPU usage fall down to 7-20% of CPU. This is strange. I'm just writing a text. Not playing WebGL game, not watching video! On Lenovo: CPU alse used up to 100% (one of two present CPU cores) but without mentioned UI lags, or just when 30-40 tabs are open, or some other app eating CPU (Netbeans, PyCharm, or something else) I've tried to move my profile directory into RAM-disk, with help of profile-sync-daemon (which moves FF profile to a RAM-disk, and sync it back to HDD from time to time with rsync), so, if the problem was in by HDD or disk subsystem - it should be solved. But there were no noticeable result. Lags of UI stay where they are. Also, I had problem with very slow and jerky page scrolling, but after I made firefox use hardware acceleration (it was disabled by blacklisting), the pages now are scrolling almost smooth. Also, I've used about:preferences, and find out the most CPU-hungry tabs and addons. It was Synthing's web interface, and Lasarus Form Save extension. Other ones was randomly displayed as "may slowdown firefox", but used just 1-2% of CPU. Also, my PC is working fine in all other apps, they starting fast, running smooth, and I have no problems with them. I don't understand how this possible, and almost despaired on this problem. I'm a web-programmer, and web browser is my working instrument, all this extensions are required to make comfortable working environment. I've just wanted to make it work faster on new laptop, but achieved the opposite result. P.S. The single thing, that I didn't tried: after I moved to Acer, I've installed firefox pakage from debian mozilla team, but on Lenovo I have firefox-esr package, with is a transition package from iceweasel. I'll try to install firefox-esr instead of plain firefox, and maybe it'll solve some problems.